This chapter introduces the task of modelling a wind farm. Different turbine and flow models are presented. Data-driven techniques previously used within the context of farm modelling are considered. The core technique used in this book, Dynamic Mode Decomposition, is discussed in great detail. Its variants suited for the purpose of modelling systems where inputs and outputs are considered, suited for the purposes of control, are presented. In addition, variants of these techniques motivated by mathematical concepts from non-linear dynamic systems theory are discussed. This chapter focuses heavily on the data-driven techniques and presents them from a mathematical and algorithmic point of view.