This thesis is about the classification of simple Lie algebras. We start with some basic structure theory given in Chapter 2. Thereafter in Chapters 3 to 5 we give the complete classification over a field of characteristic zero. In Chapter 6 we discuss the notion ofChevalley basis which is important for the definition of classical Lie algebras over a field of positive characteristic. We end up in Chapter 7 with providing the basic notions about the positive characteristic case as well as stating the latest results on this matter.