DeclareDesign is statistical software to aid researchers in characterizing and diagnosing research designs — including experiments, quasi-experiments, and observational studies. DeclareDesign consists of a core package, as well as three companion packages that stand on their own but can also be used to complement the core package: randomizr: Easy-to-use tools for common forms of random assignment and sampling; fabricatr: Tools for fabricating data to enable frontloading analysis decisions in social science research; estimatr: Fast estimators for social science research.