Water scarcity is becoming a significant problem in many countries and is linked to a complex mix of demographic, environmental, economic, and social issues. In a recent review of the literature, Michie and her colleagues identified 83 theories relevant to behaviour change. This chapter reviews five of these theories that are particularly relevant for household water conservation. Focus Theory of Normative Conduct differentiates between two kinds of social norms: injunctive norms, and descriptive norms. The chapter describes the main elements of behaviour change projects, and how the Behaviour Change Wheel (BCW) can help improve the quality of interventions and policies related to domestic water use. The Capability, Opportunity, Motivation – Behaviour (COM-B) system is an overarching, integrative model of behaviour that lies at the hub of the BCW. COM-B can help water policy makers and behaviour change practitioners understand behaviour in context by identifying the main causes of problematic behaviours, such as the failure to adopt water-saving practices.