AD Model Builder, or ADMB, is a powerful software package for the development of state-of-the-art nonlinear statistical models. ADMB is built around the AUTODIF Library, a C++ language extension which implements reverse mode automatic differentiation. A closely related software package, ADMB-RE, implements random effects in nonlinear models. ADMB was created by David Fournier and now continues to be developed by the ADMB Project, a creation of the non-profit ADMB Foundation. ADMB is free, open source, and available for Windows, Linux, MacOS, and Sun/SPARC.