This article will concentrate on the treatment of endogenous growth by neo-classical growth theorists in order to explain why the endogenous growth approach was not incorporated into the neo-classical growth program until the late 1980s, although the essential features were well known during the 1960s. In order to assess the transformations inside the neoclassical growth approach, an useful methodological framework is the Methodology of Scientific Research Programs (MRSP) proposed by a Popperian scholar, Imre Lakatos (1970). The Lakatos’s perspective on the concept of scientific progress did have an relevant impact on economic methodology and there are Lakatosian case studies on every areas of economic thought ( Hands, 2001, p. 287). Even if the MSRP is not without its critics among economic methodologists it remains a useful framework within which to analyse the evolution of economic ideas (Backhouse, 1998). The thesis which results is that the new (endogenous) growth theory may be seen in terms of an extension of the neo-classical research programme in the sense of Lakatos to incorporate theoretical elements which previously fell beyond its scope.