This chapter provides a detailed review of trust theories mostly from the perspective of social scientists. It covers what constitutes trust, how it evolves and measures which can potentially encourage its development. The chapter describes the different types and definitions of trust, and the alternative influences and effects of each. It discusses the theoretical benefits of trust in different scenarios, alongside potential barriers to creation and its fragile and dynamic nature. The chapter outlines the theoretical composition of trust, including constructs and attributes. Mistrust could also be generated by contractors in submitting claims for additional monies which are deemed as being optimistic and unsubstantiated. 'System-based' trust is based on faith in systems and related to formal procedural and contractual arrangements and policies that shape and develop organisational relationships. 'Cognition-based' trust is related to knowledge, understanding, communication, and interaction. Trust is a multi-dimensional construct that plays an important part in reducing opportunism and improving cooperative behaviours.