diff --git a/README.md b/README.md
index e17bf57..39f95e9 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,7 @@ Alchina is a Machine Learning framework.
 
 **Clusters**
 
-- K-Means clustering
+- K-Means
 
 **Optimizers**
 
diff --git a/alchina/classifiers/__init__.py b/alchina/classifiers/__init__.py
new file mode 100644
index 0000000..636d3bc
--- /dev/null
+++ b/alchina/classifiers/__init__.py
@@ -0,0 +1,6 @@
+"""Classifiers."""
+
+from .linear import LinearClassifier, RidgeClassifier
+from .knn import KNNClassifier
+
+__all__ = ["LinearClassifier", "RidgeClassifier", "KNNClassifier"]
diff --git a/alchina/classifiers.py b/alchina/classifiers/linear.py
similarity index 89%
rename from alchina/classifiers.py
rename to alchina/classifiers/linear.py
index dcef451..7ed9b76 100644
--- a/alchina/classifiers.py
+++ b/alchina/classifiers/linear.py
@@ -1,18 +1,18 @@
-"""Classifiers."""
+"""Linear Classifiers."""
 
 import numpy as np
 
 from abc import ABC, abstractmethod
 
-from .exceptions import InvalidInput, NotFitted
-from .metrics import accuracy_score
-from .optimizers import GradientDescent
-from .preprocessors import Standardization
-from .utils import check_dataset_consistency, features_reshape
+from alchina.exceptions import InvalidInput, NotFitted
+from alchina.metrics import accuracy_score
+from alchina.optimizers import GradientDescent
+from alchina.preprocessors import Standardization
+from alchina.utils import check_dataset_consistency, features_reshape
 
 
-class AbstractClassifier(ABC):
-    """Abstract class for classifiers algorithms."""
+class AbstractLinearClassifier(ABC):
+    """Abstract class for linear classifiers algorithms."""
 
     def __init__(self, *args, optimizer=None, standardize: bool = True, **kwargs):
         self.standardize = Standardization() if standardize else None
@@ -88,7 +88,7 @@ def score(self, X, y):
         return accuracy_score(self.predict(X), y)
 
 
-class LinearClassifier(AbstractClassifier):
+class LinearClassifier(AbstractLinearClassifier):
     """Linear classifier (logistic regressor)."""
 
     def sigmoid(self, z):
@@ -111,7 +111,7 @@ def gradient(self, X, y, theta):
         return X.T.dot(self.hypothesis(X, theta) - y)
 
 
-class RidgeClassifier(LinearClassifier):
+class RidgeClassifier(AbstractLinearClassifier):
     """Regularized linear classifier."""
 
     def __init__(self, *args, regularization: float = 1, **kwargs):
diff --git a/alchina/clusters/__init__.py b/alchina/clusters/__init__.py
new file mode 100644
index 0000000..d9ce662
--- /dev/null
+++ b/alchina/clusters/__init__.py
@@ -0,0 +1,5 @@
+"""Clusters."""
+
+from .kmeans import KMeans
+
+__all__ = ["KMeans"]
diff --git a/alchina/clusters.py b/alchina/clusters/kmeans.py
similarity index 97%
rename from alchina/clusters.py
rename to alchina/clusters/kmeans.py
index d10784f..5a4c073 100644
--- a/alchina/clusters.py
+++ b/alchina/clusters/kmeans.py
@@ -1,10 +1,10 @@
-"""Clusters."""
+"""K-Means."""
 
 import numpy as np
 
 
 class KMeans(object):
-    """K-means algorithm."""
+    """K-Means algorithm."""
 
     def __init__(self, n_centroids, max_iterations=300):
         self.n_centroids = n_centroids
diff --git a/alchina/optimizers/__init__.py b/alchina/optimizers/__init__.py
new file mode 100644
index 0000000..6942544
--- /dev/null
+++ b/alchina/optimizers/__init__.py
@@ -0,0 +1,5 @@
+"""Optimizers."""
+
+from .gradient_descent import GradientDescent, SGD, MBGD
+
+__all__ = ["GradientDescent", "SGD", "MBGD"]
diff --git a/alchina/optimizers.py b/alchina/optimizers/gradient_descent.py
similarity index 93%
rename from alchina/optimizers.py
rename to alchina/optimizers/gradient_descent.py
index 5a24c79..49fea77 100644
--- a/alchina/optimizers.py
+++ b/alchina/optimizers/gradient_descent.py
@@ -1,15 +1,15 @@
-"""Optimizers algorithms."""
+"""Gradient Descent Optimizers algorithms."""
 
 import numpy as np
 
 from abc import ABC, abstractmethod
 from typing import Optional
 
-from .exceptions import InvalidInput, NotBuilt
-from .utils import check_dataset_consistency, shuffle_dataset
+from alchina.exceptions import InvalidInput, NotBuilt
+from alchina.utils import check_dataset_consistency, shuffle_dataset
 
 
-class AbstractOptimizer(ABC):
+class AbstractGDOptimizer(ABC):
     """Abstract class for optimizers algorithms."""
 
     def __init__(self, iterations: int = 100, history: bool = False):
@@ -32,7 +32,7 @@ def __call__(self):
         pass  # pragma: no cover
 
 
-class GradientDescent(AbstractOptimizer):
+class GradientDescent(AbstractGDOptimizer):
     """Batch gradient descent."""
 
     def __init__(self, *args, learning_rate: float = 0.01, **kwargs):
@@ -60,7 +60,7 @@ def __call__(self, X, y, *args, **kwargs):
         return self.parameters
 
 
-class SGD(AbstractOptimizer):
+class SGD(AbstractGDOptimizer):
     """Stochastic gradient descent."""
 
     def __init__(
@@ -92,7 +92,7 @@ def __call__(self, X, y, *args, **kwargs):
         return self.parameters
 
 
-class MBGD(AbstractOptimizer):
+class MBGD(AbstractGDOptimizer):
     """Mini-batch gradient descent."""
 
     def __init__(
diff --git a/alchina/preprocessors/__init__.py b/alchina/preprocessors/__init__.py
new file mode 100644
index 0000000..8d53785
--- /dev/null
+++ b/alchina/preprocessors/__init__.py
@@ -0,0 +1,6 @@
+"""Preprocessors."""
+
+from .data import Normalization, Standardization
+from .pca import PCA
+
+__all__ = ["Normalization", "Standardization", "PCA"]
diff --git a/alchina/preprocessors/data.py b/alchina/preprocessors/data.py
new file mode 100644
index 0000000..81da89b
--- /dev/null
+++ b/alchina/preprocessors/data.py
@@ -0,0 +1,41 @@
+"""Data Preprocessors."""
+
+import numpy as np
+
+from typing import Optional
+
+
+class Normalization(object):
+    """Rescale the data via a normalization.
+
+    Produce:
+    - Bring all values into the range [0, 1]
+    """
+
+    def __call__(self, X, axis: int = 0):
+        min_x = np.amin(X, axis=axis)
+        max_x = np.amax(X, axis=axis)
+        return (X - min_x) / (max_x - min_x)
+
+
+class Standardization(object):
+    """Rescale the data via a standardization
+
+    Produce:
+    - mean(Xstandardized) = 0
+    - std(Xstandardized) = 1
+    """
+
+    def __init__(self, mu: Optional[int] = None, sigma: Optional[int] = None):
+        self.mu = mu
+        self.sigma = sigma
+
+    def __call__(self, X, axis: int = 0):
+        if self.mu is None or self.sigma is None:
+            self.mu = np.mean(X, axis=axis)
+            self.sigma = np.std(X, axis=axis)
+
+            if not np.any(self.sigma):
+                self.sigma = np.ones_like(self.sigma)
+
+        return np.divide(X - self.mu, self.sigma)
diff --git a/alchina/preprocessors.py b/alchina/preprocessors/pca.py
similarity index 64%
rename from alchina/preprocessors.py
rename to alchina/preprocessors/pca.py
index c4995a6..3f09e4f 100644
--- a/alchina/preprocessors.py
+++ b/alchina/preprocessors/pca.py
@@ -1,51 +1,15 @@
-"""Preprocessors."""
+"""Principal Component Analysis."""
 
 import numpy as np
 
 from typing import Optional
 
-from .exceptions import InvalidInput, NotFitted
-from .utils import features_reshape
-
-
-class Normalization(object):
-    """Rescale the data via a normalization.
-
-    Produce:
-    - Bring all values into the range [0, 1]
-    """
-
-    def __call__(self, X, axis: int = 0):
-        min_x = np.amin(X, axis=axis)
-        max_x = np.amax(X, axis=axis)
-        return (X - min_x) / (max_x - min_x)
-
-
-class Standardization(object):
-    """Rescale the data via a standardization
-
-    Produce:
-    - mean(Xstandardized) = 0
-    - std(Xstandardized) = 1
-    """
-
-    def __init__(self, mu: Optional[int] = None, sigma: Optional[int] = None):
-        self.mu = mu
-        self.sigma = sigma
-
-    def __call__(self, X, axis: int = 0):
-        if self.mu is None or self.sigma is None:
-            self.mu = np.mean(X, axis=axis)
-            self.sigma = np.std(X, axis=axis)
-
-            if not np.any(self.sigma):
-                self.sigma = np.ones_like(self.sigma)
-
-        return np.divide(X - self.mu, self.sigma)
+from alchina.exceptions import InvalidInput, NotFitted
+from alchina.utils import features_reshape
 
 
 class PCA(object):
-    """Principal Component Analysis."""
+    """Principal Component Analysis algorithm."""
 
     def __init__(self, n_components: Optional[int] = None):
         self.n_components = n_components
diff --git a/alchina/regressors/__init__.py b/alchina/regressors/__init__.py
new file mode 100644
index 0000000..636e8d9
--- /dev/null
+++ b/alchina/regressors/__init__.py
@@ -0,0 +1,5 @@
+"""Regressors."""
+
+from .linear import LinearRegressor, RidgeRegressor
+
+__all__ = ["LinearRegressor", "RidgeRegressor"]
diff --git a/alchina/regressors.py b/alchina/regressors/linear.py
similarity index 90%
rename from alchina/regressors.py
rename to alchina/regressors/linear.py
index a0b09e1..86d11d1 100644
--- a/alchina/regressors.py
+++ b/alchina/regressors/linear.py
@@ -4,14 +4,14 @@
 
 from abc import ABC, abstractmethod
 
-from .exceptions import InvalidInput, NotFitted
-from .metrics import r2_score
-from .optimizers import GradientDescent
-from .preprocessors import Standardization
-from .utils import check_dataset_consistency, features_reshape
+from alchina.exceptions import InvalidInput, NotFitted
+from alchina.metrics import r2_score
+from alchina.optimizers import GradientDescent
+from alchina.preprocessors import Standardization
+from alchina.utils import check_dataset_consistency, features_reshape
 
 
-class AbstractRegressor(ABC):
+class AbstractLinearRegressor(ABC):
     """Abstract class for regressors algorithms."""
 
     def __init__(self, *args, optimizer=None, standardize: bool = True, **kwargs):
@@ -71,7 +71,7 @@ def score(self, X, y):
         return r2_score(self.predict(X), y)
 
 
-class LinearRegressor(AbstractRegressor):
+class LinearRegressor(AbstractLinearRegressor):
     """Linear regressor."""
 
     def hypothesis(self, X, theta):
@@ -95,7 +95,7 @@ def normal(self, X, y):
         self.optimizer.parameters = np.linalg.pinv(X.T.dot(X)).dot(X.T).dot(y)
 
 
-class RidgeRegressor(AbstractRegressor):
+class RidgeRegressor(AbstractLinearRegressor):
     """Ridge regressor."""
 
     def __init__(self, *args, regularization: float = 1, **kwargs):
diff --git a/examples/clusters.py b/examples/clusters.py
index 52ad655..82f036a 100644
--- a/examples/clusters.py
+++ b/examples/clusters.py
@@ -23,9 +23,6 @@
 kmeans = KMeans(n_centroids=2)
 kmeans.fit(X)
 
-# Display the number of model iterations
-print(f"K-Means iterations : {kmeans.iterations}")
-
 # Plot the results
 A = X[np.array(kmeans.indexes) == 0]
 B = X[np.array(kmeans.indexes) == 1]
diff --git a/examples/selection.py b/examples/metrics.py
similarity index 100%
rename from examples/selection.py
rename to examples/metrics.py
diff --git a/tests/test_classifiers.py b/tests/classifiers/test_linear_classifiers.py
similarity index 98%
rename from tests/test_classifiers.py
rename to tests/classifiers/test_linear_classifiers.py
index f9471b4..7d6bea6 100644
--- a/tests/test_classifiers.py
+++ b/tests/classifiers/test_linear_classifiers.py
@@ -1,4 +1,4 @@
-"""Classifiers tests."""
+"""Linear Classifiers tests."""
 
 import numpy as np
 import pytest
@@ -75,7 +75,7 @@ def test_linear_classifier_history_disabled():
 
 
 def test_linear_classifier_multiclass():
-    """Test of `LinearClassifier` with no multiclass."""
+    """Test of `LinearClassifier` with multiclass."""
     lc = LinearClassifier(learning_rate=0.1, iterations=2)
 
     X = np.array([[0], [1], [2]])
diff --git a/tests/test_clusters.py b/tests/clusters/test_kmeans.py
similarity index 95%
rename from tests/test_clusters.py
rename to tests/clusters/test_kmeans.py
index cbf7af6..f0c1937 100644
--- a/tests/test_clusters.py
+++ b/tests/clusters/test_kmeans.py
@@ -1,4 +1,4 @@
-"""Clusters tests."""
+"""K-Means tests."""
 
 import numpy as np
 
diff --git a/tests/test_optimizers.py b/tests/optimizers/test_gradient_descent.py
similarity index 98%
rename from tests/test_optimizers.py
rename to tests/optimizers/test_gradient_descent.py
index 547e98c..3705b23 100644
--- a/tests/test_optimizers.py
+++ b/tests/optimizers/test_gradient_descent.py
@@ -1,4 +1,4 @@
-"""Optimizers tests."""
+"""Gradient Descent Optimizers tests."""
 
 import pytest
 import numpy as np
diff --git a/tests/preprocessors/test_data.py b/tests/preprocessors/test_data.py
new file mode 100644
index 0000000..ed47318
--- /dev/null
+++ b/tests/preprocessors/test_data.py
@@ -0,0 +1,33 @@
+"""Data Preprocessors tests."""
+
+import numpy as np
+import pytest
+
+from alchina.preprocessors import Normalization, Standardization
+
+
+# --- Normalization ---
+
+
+def test_normalization():
+    """Test of `Normalization` class."""
+    normalize = Normalization()
+
+    X = np.array([1, 10, 100, 5, 0.01])
+    X_norm = normalize(X)
+
+    assert np.all((X_norm >= 0) & (X_norm <= 1))
+
+
+# --- Standardization ---
+
+
+def test_standardization():
+    """Test of `Standardization` class."""
+    standardize = Standardization()
+
+    X = np.array([1, 10, 100, 5, 0.01])
+    X_stand = standardize(X)
+
+    assert np.mean(X_stand) == pytest.approx(0)
+    assert np.std(X_stand) == pytest.approx(1)
diff --git a/tests/test_preprocessors.py b/tests/preprocessors/test_pca.py
similarity index 64%
rename from tests/test_preprocessors.py
rename to tests/preprocessors/test_pca.py
index 2d5f7df..4795a27 100644
--- a/tests/test_preprocessors.py
+++ b/tests/preprocessors/test_pca.py
@@ -1,37 +1,10 @@
-"""Preprocessors tests."""
+"""Principal Component Analysis tests."""
 
 import numpy as np
 import pytest
 
 from alchina.exceptions import InvalidInput, NotFitted
-from alchina.preprocessors import Normalization, Standardization, PCA
-
-
-# --- Normalization ---
-
-
-def test_normalization():
-    """Test of `Normalization` class."""
-    normalize = Normalization()
-
-    X = np.array([1, 10, 100, 5, 0.01])
-    X_norm = normalize(X)
-
-    assert np.all((X_norm >= 0) & (X_norm <= 1))
-
-
-# --- Standardization ---
-
-
-def test_standardization():
-    """Test of `Standardization` class."""
-    standardize = Standardization()
-
-    X = np.array([1, 10, 100, 5, 0.01])
-    X_stand = standardize(X)
-
-    assert np.mean(X_stand) == pytest.approx(0)
-    assert np.std(X_stand) == pytest.approx(1)
+from alchina.preprocessors import PCA
 
 
 # --- PCA ---
diff --git a/tests/test_regressors.py b/tests/regressors/test_linear_regressors.py
similarity index 99%
rename from tests/test_regressors.py
rename to tests/regressors/test_linear_regressors.py
index 5a05615..41005ea 100644
--- a/tests/test_regressors.py
+++ b/tests/regressors/test_linear_regressors.py
@@ -1,4 +1,4 @@
-"""Regressors tests."""
+"""Linear Regressors tests."""
 
 import numpy as np
 import pytest