The software package VITESS for the simulation of neutron scattering instruments is presented. The concept, the main features and the use of the program are described. The new features of version 2.5 are explained in detail. A survey of the existing modules is given. Particular emphasis is given to modules that are used to simulate optical components, such as 'supermirror ensemble', 'monochromator/analyzer' or 'bender'. Examples of simulations of existing and planned instruments are presented, e.g. a powder diffractometer installed on a long pulse target station using wavelength frame multiplication. Simulation results of the existing instrument HET at ISIS are compared to experimental results. Furthermore, a view to the coming version 2.6 is presented. One of its features is discussed in detail: It will be possible to optimize parts of the instruments numerically. As a first application, focusing guides were optimized. General results of this method as well as results for a special instrument are shown. Finally, a general outlook to the planned developments of VITESS is presented.