Methods of optimisation are frequently applied in process engineering. Parameter identification and optimisation of complete chemical plants are only two examples. Methods of linear and nonlinear programming, with and without constraints, will be presented. Limitation to methods proven in practice and a wide distribution will be done. Problems of global optimisation will be discussed. Out of the heuristic algorithms the Linnhoff-pinch-method will be mentioned. Finally, some applications of optimisation in process engineering will be outlined.