Steadily rising energy demand due to an increasing population requires the application of the best available technologies: both because energy resources are limited and because requirements for the protection of the environment have become stricter. This article deals with computer models for the optimisation of energy conversion plants for different applications. For new power plants, optimal design has to be defined whereas for existing plants, the performance by changing operational parameters is more interesting. The methods presented consider design as well as operational aspects. Examples of some relevant projects are described for illustration.