When we want to study the behaviour of the economy in the long-run term, we can no longer focus on the fluctuations but on growth instead. After studying the evolution of the output per capita of some rich countries, economists have come out with three main unexplained facts: (1) Growth has been very strong during the last 50 years, (2) Growth has slowed down since the mid-1970’s and (3) Output per capita among those countries has converged over time. Although we cannot take the study of a few rich countries as significant, we can use it to find out the main focus of attention of the growth models that will derive: (1) A theory that explains growth must also be able to explain the lack of growth (either in other periods of time or in other countries), (2) Convergence among countries and (3) It is not so much the lower growth that needs to be explained but the periods of fast growth experienced by some countries