This chapter introduces the theory of growth and accumulation of Nobuo Okishio and to compare it with other theories. It needs some yardstick to evaluate the functioning of a variety of models and proposes a measure by which make such comparisons. The neoclassical model of growth and the neo-Kaldorian model are familiar among analytical political economists in the English-speaking countries. The chapter contrasts Okishio's theory with these models and discusses the contemporary significance of Okishio's theoretical approach and that of others in the tradition of political economy and/or heterodox economics. The gist of Okishio's theory of accumulation is to clarify the necessity of cyclical movement in a capitalist economy, using an analysis based on his Harrod-Okishio type of investment function. The aim of political economy is opening-up economics, that is to examine issues and deploying approaches neglected by the current orthodoxy, demonstrating the vitality of heterodox economics and its continued relevance to contemporary social questions.