synthACS provides four main features. Firstly, it provides a wrapper to library(acs) to access numerous American Community Survey (ACS) base tables which may be of interest to many researchers. Secondly, it builds synthetic microdatasets of ACS data (pulled via API) at any specified geographic level with 10 default individual attributes. Thirdly, synthACS provides funtionality for users to add additional ACS & non-ACS synthetic data-attributes to micro-datasets based on macro population characteristics. And finally, in addition to creating synthetic data, synthACS also conducts spatial microsimulation modeling (SMSM) (ie- optimally fits synthetic microdata to macrodata constraints) via simulated annealing. SMSM is conducted in parallel by default.