SAAM II is a modeling, simulation, and analysis software package, which supports the development and statistical calibration of compartmental models in biological, metabolic, and pharmaceutical systems. Used worldwide by more than 7,000 pharmaceutical, biomedical and bioengineering professionals – with citations in over 2,000 scientific publications, SAAM II is a proven and powerful tool for research, development, and teaching. Additionally, SAAM II’s GUI is easy to learn and easy to use. Thus – regardless of level of modeling expertise – users can define models, run simulations, and analyze results quickly. Under the hood, SAAM II employs state-of-the-art numerical and statistical methods and algorithms. It is widely regarded as the most robust and accurate software package for solving systems of differential equations and for fitting model parameters to experimental data sets with specified (and flexible) error models. In addition, PopKinetics analyzes populations of individual subjects, where each subject in the population is represented by one SAAM II Compartmental Study file. PopKinetics is compatible with SAAM II v2.3 and higher.