An increasing number of public institutional investors are adopting sustainable and ethical investment policies. While financial tests of materiality and norm structures are often assumed to guide their implementation, this assumption is challenged by the increasing complexity in global financial markets. This article provides an analytical framework to explain these implementation problems by drawing attention to the ambiguity inherent in investment policies. Ambiguity means there is no ideal outcome. Agents must use their discretion to interpret investment policies, which is at odds with conventional theories of discretion that assume a unique policy goal. This article argues that ambiguity impacts institutional investors in two contrasting ways. Ambiguity acts as a built-in mechanism for adapting investment policies to increasing complexity in global financial markets. But the resources required to maintain legitimacy under ambiguity detract from the investor's capacity to actually implement its policy. This framework is used to analyze the evolution of the Norwegian sovereign wealth fund (SWF)'s ethical investment policy. The article finds that agents use their discretion to interpret the Fund's investment policy in ways that align with its long-term mandate.