In this chapter, written for the book 'Law and Method. Interdisciplinary Research info Law', we describe the methodology of Law and Economics. We start by discussing the mainstream economic approach to law, including the topics of rationality, the use of (sometimes unrealistic) assumptions, the distinction between positive and normative Law and Economics,and the concept of efficiency. Subsequently we describe the viewpoint that law can be seen as a way to correct different types of market failures (market power, externalities, collective goods and information asymmetry). Then we analyze the case for regulatory intervention and its limits, spending attention to the Coase theorem and the importance of transaction costs. Next we discuss insights from Behavioural Law and Economics and in our conclusion state why Law and Economics is useful to lawyers.