DyOS is a software tool for the solution of dynamic optimization problems. This problem class includes the optimization of chemical plant design and operation, and a large variety of related applications. A large class of dynamic optimization problems involve models given by differential-algebraic equations. DyOS allows the model to be accessed via a so-called Equation Set Object (ESO) interface, a standard developed in the Cape-Open project. The commercial modeling and simulation package gPROMS, for example, can be used as a model server for DyOS. DyOS features state-of-the-art numerical solution methods, among them an automatic adaptation of the control profile discretization . DyOS has successfully been used to solve multi-stage problems, mixed integer dynamics optimization problems, dynamic parameter and input estimations, and optimal experimental design problems.