FreeDyn is a free simulation software designed for solving challenging scientific and industrial problems in multibody dynamics such as inverse problems with systems consisting of flexible bodies. It serves as an easy-to-use software tool for modeling mechanical systems including rigid as well as flexible bodies connected by joints and constraints. The geometry can either be imported from a CAD system or designed in FreeDyn from scratch. FreeDyn solves the equations for dynamic simulations and offers an advanced visualization of solutions by graphs, plots and animations.