Tinker, stylized as TINKER, is a computer software application for molecular dynamics simulation with a complete and general package for molecular mechanics and molecular dynamics, with some special features for biopolymers. The core of the package is a modular set of callable routines which allow manipulaing coordinates and evaluating potential energy and derivatives via straightforward means. Tinker works on Windows, OS X, Linux and Unix. The source code is available free of charge under a restrictive license. The code is written in portable FORTRAN 77 with common extensions, and some C.