In many countries, both pay-as-you-go (PAYG) and funding are used to finance pensions, although the balance between the two principles differs a lot between countries. Over the last decades, many countries made a gradual transition to more funding. In this paper, we develop an analytical framework that includes three models of pension design, allowing us to study the role of efficiency aspects, redistributional aspects and political-economy aspects. We subsequently analyze the impact of several trends (a permanent decline in the rate of return on financial markets, a decline in the average rate of economic growth, decreased output volatility and increased capital market volatility) on the optimal balance between PAYG and funding. We argue that it may be optimal to revise the gradual transition to more funding and to revive PAYG.