In many modelling applications, finding optimal solutions – typically in a vast and complex solution space – is a core task. This chapter will provide an introduction to selected mathematical models and methods, which are available to address selected optimisation problems. To begin with, some exemplary optimisation tasks are described and approaches that have been established to solve these will be introduced. In a next step, optimisation algorithms for linear and non-linear problem formulations are briefly discussed. The central part of the chapter discusses the design and application of Genetic Algorithms (GAs), which have been successfully applied to solve a variety of environmental and ecological modelling problems. A specific applica...