Positive psychological capital (PPC) provides a framework that enables scholars to extend current research into positive organisational behaviour (POB). The construct, which comprises self-efficacy, hope, optimism, and resiliency, was introduced by Luthans, Luthans, and Luthans (2004) as a measure comparable and complimentary to measures of human (or intellectual), social and traditional physical capital. This paper explores the rationale behind positive psychological capital, reviewing the model with a specific focus on the clarifying the distinctive value of component constructs, and further, on clarifying the criteria by which additional states might be assessed for inclusion in the theory; trust is briefly assessed under these criteria as a potential addition to the model. To conclude, an agenda for future research is outlined.