This paper studies theoretically and experimentally the distinctive effects of risk and ambiguity on willingness to wait. We analyze a simple real options model in which a decision maker chooses the timing of investment and can observe the environment to learn about it. The model predicts that higher risk delays investment. Higher ambiguity accelerates investment if decision makers are ambiguity averse, while a delay of investment indicates an ambiguity seeking attitude. Experimental data reveal that willingness to wait increases in both risk and ambiguity. This confirms the predicted effect of risk and implies that subjects are ambiguity seeking. The last finding is in contrast with aversion towards ambiguity often found in static willingness-to-pay experiments.