nlmixr is a free and open source R package for fitting nonlinear pharmacokinetic (PK), pharmacodynamic (PD), joint PK/PD and quantitative systems pharmacology (QSP) mixed-effects models. Currently, nlmixr is capable of fitting both traditional compartmental PK models as well as more complex models implemented using ordinary differential equations (ODEs). It is under intensive development and has succeeded in attracting extensive attention and a willingness to make contributions from the pharmaceutical modeling community. We believe that, over time, it will become a capable, credible alternative to commercial software tools, such as NONMEM, Monolix, and Phoenix NLME.