The concept of psychological capital (PsyCap) has attracted a great deal of interest from both academics and practitioners and has been linked to employee attitudes, behavior and performance at different levels of analysis. Yet, the nature of the concept, its measurement, the factors that influence its development, and when and how it influences individual-level, team-level and organizational-level outcomes are the subject of continued debate in the literature. This article offers a detailed and focused review of the existing literature on PsyCap, with the aim of developing an agenda for future research. In particular, we call for researchers to pay greater attention to possible multi-level applications of PsyCap research, examine the underlying mechanisms by which PsyCap influences individual-level, team-level and organizational-level outcomes, and identify possible factors that may moderate the relationship between PsyCap and its outcomes. In doing this, we provide a roadmap for scholars to progress the development of the field.