This chapter aims to give an account of Lakatos’ contribution to the philosophy of mathematics. Its main thesis is that Lakatos’ most important contribution was to introduce the historical approach to the philosophy of mathematics. When Lakatos published his paper ‘Proofs and Refutations’ in 1963–1964, the historical approach has been used in the philosophy of science for 123 years. This poses the problem of why there was such a long delay between the introduction of the historical approach in the philosophy of science and its introduction in the philosophy of mathematics. The chapter presents some possible explanations of why this was so, and what enabled Lakatos to make his innovation. Some details are then given of Lakatos’ work in philosophy of mathematics to show the striking and influential results to which his new historical approach led.