This article explores one of the most significant problems of contemporary global capitalism, namely 'secular stagnation', in the light of theories of the three different schools of economic thought (Neo-classical, Post-Keynesian and Radical Political Economy). In the age of stagnation, the ability of global capitalist system to maintain its stability and survival has declined significantly. The long-run stagnation has weakened the consensus among social classes, leading to a 'political crisis' and an increasing authoritarian tendency in contemporary capitalism. This paper divides modern orthodox theoretical explanations of stagnation into three categories: demand-sided theories (Summers/Krugman), supply-sided ones (Gordon) and theories denying the existence of stagnation (Bernanke & Rogoff). Although some leading orthodox economists accept that there has been a strong tendency of stagnation in advanced capitalist world since the start of this century, they neither consider stagnation to be a consequence of capitalist accumulation process nor one of neo-liberal variant of capitalism. In contrast, analyzing stagnation from a wider perspective with historical, social and political dimensions, heterodox theories have stronger theoretical foundations than those of orthodox theories. While Post-Keynesians argue that stagnation stems from structural/chronic demand deficiency caused by neo-liberalism enhancing income inequalities, radical political economists emphasize that stagnation is a 'systemic' problem inherent to monopoly, mature capitalism.