diff --git a/src/grid.rs b/src/grid.rs
index 79b44e1..3c77d04 100644
--- a/src/grid.rs
+++ b/src/grid.rs
@@ -21,3 +21,48 @@ impl Grid {
         self.cells[y * self.width + x] = v;
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn new_initializes_to_zero() {
+        let grid = Grid::new(4, 3);
+        assert_eq!(grid.width, 4);
+        assert_eq!(grid.height, 3);
+        assert!(grid.cells.iter().all(|&v| v == 0.0));
+    }
+
+    #[test]
+    fn new_allocates_correct_cell_count() {
+        let grid = Grid::new(5, 7);
+        assert_eq!(grid.cells.len(), 35);
+    }
+
+    #[test]
+    fn set_and_get_roundtrip() {
+        let mut grid = Grid::new(4, 4);
+        grid.set(2, 3, 1.5);
+        assert_eq!(grid.get(2, 3), 1.5);
+    }
+
+    #[test]
+    fn set_uses_row_major_indexing() {
+        let mut grid = Grid::new(4, 4);
+        grid.set(1, 2, 9.9);
+        // y * width + x = 2 * 4 + 1 = 9
+        assert_eq!(grid.cells[9], 9.9);
+    }
+
+    #[test]
+    fn set_does_not_clobber_neighbors() {
+        let mut grid = Grid::new(3, 3);
+        grid.set(0, 0, 1.0);
+        grid.set(1, 0, 2.0);
+        grid.set(0, 1, 3.0);
+        assert_eq!(grid.get(0, 0), 1.0);
+        assert_eq!(grid.get(1, 0), 2.0);
+        assert_eq!(grid.get(0, 1), 3.0);
+    }
+}
diff --git a/src/renderer.rs b/src/renderer.rs
index d6ebe41..86009d2 100644
--- a/src/renderer.rs
+++ b/src/renderer.rs
@@ -42,3 +42,46 @@ pub fn render(grid: &Grid, time: f32, stdout: &mut impl Write) -> std::io::Resul
 
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn lava_color_zero_is_black() {
+        let Color::Rgb { r, g, b } = lava_color(0.0) else {
+            panic!("expected Rgb variant");
+        };
+        assert_eq!((r, g, b), (0, 0, 0));
+    }
+
+    #[test]
+    fn lava_color_clamps_large_input() {
+        // v=100 * 0.35 = 35, clamped to 1.0 → same as v where v*0.35 == 1.0
+        let Color::Rgb { r: r1, g: g1, b: b1 } = lava_color(100.0) else {
+            panic!("expected Rgb variant");
+        };
+        let Color::Rgb { r: r2, g: g2, b: b2 } = lava_color(3.0) else {
+            panic!("expected Rgb variant");
+        };
+        assert_eq!((r1, g1, b1), (r2, g2, b2));
+    }
+
+    #[test]
+    fn lava_color_red_dominates_at_low_values() {
+        // At low (but nonzero) t, red > green > blue due to polynomial degrees
+        let Color::Rgb { r, g, b } = lava_color(1.0) else {
+            panic!("expected Rgb variant");
+        };
+        assert!(r >= g);
+        assert!(g >= b);
+    }
+
+    #[test]
+    fn render_writes_output() {
+        let grid = Grid::new(4, 4);
+        let mut buf: Vec<u8> = Vec::new();
+        render(&grid, 0.0, &mut buf).unwrap();
+        assert!(!buf.is_empty());
+    }
+}
diff --git a/src/simulation.rs b/src/simulation.rs
index f356533..20dafd7 100644
--- a/src/simulation.rs
+++ b/src/simulation.rs
@@ -50,3 +50,92 @@ pub fn update_grid(grid: &mut Grid, balls: &[Metaball]) {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn make_ball(x: f32, y: f32, vx: f32, vy: f32, radius: f32) -> Metaball {
+        Metaball { x, y, vx, vy, radius }
+    }
+
+    // clamp_and_bounce tests (tested via update_balls)
+
+    #[test]
+    fn ball_moves_by_velocity() {
+        let mut balls = vec![make_ball(10.0, 10.0, 2.0, -3.0, 1.0)];
+        update_balls(&mut balls, 100.0, 100.0);
+        assert_eq!(balls[0].x, 12.0);
+        assert_eq!(balls[0].y, 7.0);
+    }
+
+    #[test]
+    fn ball_bounces_off_right_wall() {
+        // Place ball near right wall with positive vx so it would exceed max
+        let mut balls = vec![make_ball(98.0, 50.0, 5.0, 0.0, 2.0)];
+        update_balls(&mut balls, 100.0, 100.0);
+        // After update: x = 103 > 98 (100 - radius), so clamp to 98 and reverse vx
+        assert_eq!(balls[0].x, 98.0);
+        assert!(balls[0].vx < 0.0);
+    }
+
+    #[test]
+    fn ball_bounces_off_left_wall() {
+        let mut balls = vec![make_ball(3.0, 50.0, -5.0, 0.0, 4.0)];
+        update_balls(&mut balls, 100.0, 100.0);
+        // After update: x = -2 < 4 (radius), clamp to 4 and flip vx positive
+        assert_eq!(balls[0].x, 4.0);
+        assert!(balls[0].vx > 0.0);
+    }
+
+    #[test]
+    fn ball_bounces_off_bottom_wall() {
+        let mut balls = vec![make_ball(50.0, 97.0, 0.0, 5.0, 3.0)];
+        update_balls(&mut balls, 100.0, 100.0);
+        assert_eq!(balls[0].y, 97.0);
+        assert!(balls[0].vy < 0.0);
+    }
+
+    #[test]
+    fn ball_bounces_off_top_wall() {
+        let mut balls = vec![make_ball(50.0, 2.0, 0.0, -5.0, 3.0)];
+        update_balls(&mut balls, 100.0, 100.0);
+        assert_eq!(balls[0].y, 3.0);
+        assert!(balls[0].vy > 0.0);
+    }
+
+    #[test]
+    fn field_value_is_higher_near_ball_center() {
+        let balls = vec![make_ball(5.0, 5.0, 0.0, 0.0, 10.0)];
+        // Cell directly on ball center should have high field value
+        let near = field_value(5.0, 5.0, &balls);
+        // Cell far from ball
+        let far = field_value(50.0, 50.0, &balls);
+        assert!(near > far);
+    }
+
+    #[test]
+    fn field_value_no_balls_returns_negative_threshold() {
+        let value = field_value(0.0, 0.0, &[]);
+        assert_eq!(value, -0.8);
+    }
+
+    #[test]
+    fn update_grid_populates_all_cells() {
+        let mut grid = Grid::new(5, 5);
+        let balls = vec![make_ball(2.0, 2.0, 0.0, 0.0, 5.0)];
+        update_grid(&mut grid, &balls);
+        // At least some cells should be non-zero
+        assert!(grid.cells.iter().any(|&v| v != 0.0));
+    }
+
+    #[test]
+    fn update_grid_center_cell_higher_than_edge() {
+        let mut grid = Grid::new(10, 10);
+        let balls = vec![make_ball(5.0, 5.0, 0.0, 0.0, 10.0)];
+        update_grid(&mut grid, &balls);
+        let center = grid.get(5, 5);
+        let edge = grid.get(0, 0);
+        assert!(center > edge);
+    }
+}