Ambiguity and learning about the equity premium can simultaneously explain the low fraction of financial wealth allocated to stocks over the life cycle and the stock market participation puzzle. Individuals are ambiguous about the size of the equity premium and are averse to this ambiguity, resulting in lower stock allocations over the life cycle, consistent with the data. As agents get older, they learn about the equity premium and increase their allocation to stocks. Furthermore, I find that ambiguity leads to underdiversification, home bias, lower Sharpe ratios, and higher savings. Similar results cannot be obtained by assuming higher risk aversion.