This chapter discusses the strengths and weaknesses of deterministic models and stochastic models and describes their applicability in environmental sciences. It starts by outlining some background concepts and philosophical issues behind deterministic and stochastic views of nature. Then, it presents a summary of the most widespread methods. The differences between deterministic and stochastic modelling are illustrated by means of a real-world application in Oman. Finally, the chapter suggests some recommendations about the use of models in environmental sciences.