Water scarcity is one of the most pressing issues of our time and it is projected to increase as global demand for water surges and climate change limits fresh water availability. If we are to reduce water demand, it is essential we draw on every tool in the box. Research from the behavioural sciences demonstrates that our behaviour is strongly influenced by others. Social norms messaging, which communicates what others are doing or what behaviour is expected in a given context, has been shown to offer a cost-effective avenue to encourage proenvironmental behaviour change, and more recently, climate resilient water behaviour. The Social Identity Approach, which seeks to explain how individuals are shaped by the groups to which we belong, posits that normative messaging may be even more effective if norms are framed in reference to behaviourally-relevant groups - our ingroups. This thesis develops and tests a novel approach; an ingroup norms appeal. Utilising social identity insights, it was predicted that messages highlighting a social identity (e.g. a local community) while promoting ingroup norms favouring climate resilient water behaviour would encourage corresponding behavioural change amongst group members. Across five studies, including two large-scale experimental field trials, this thesis provides the first comprehensive empirical examination of an ingroup norms appeal in the context of water conservation. This research demonstrates that not only is the appeal effective in motivating climate resilient water behaviour, it is more efficacious than alternative message-based interventions, such as an informationonly campaign or a general social norms appeal. Mediating and moderating variables are also examined. Importantly, this research bridges the theoretical-practice gap. Research collaborations were established with industry partners and the appeal is now being utilised within the UK water sector to engage water end-users.