Recent research in investment theory emphasizes the importance of sunk investment costs, uncertainty in returns, and flexibility in investment timing. Allowing for the presence of these characteristics alters traditional discounted cash flow rules for when to invest. Those rules will recommend investing at lower rate-of-return thresholds than is optimal. This article describes this research and suggests the range of potential situations to which the theory applies. It also discusses the implications for policy analysis and suggests that government programs to encourage investment may, in some cases, be inappropriate. After discussing a wide array of possible applications, we focus on one in particular: programs to encourage energy-efficient investment. The examples suggest the importance of applying the new investment theory for economic analysis of investment in energy-efficient technologies.