Global environmental issues require substantial uptake of pro-environmental behaviours, which in turn, requires effective methods of behaviour change. The broad aim of this thesis was to investigate processes driving behavioural spillover – when performing environmental behaviours increases the adoption of other environmental behaviours. In particular, the research focuses on how different dimensions of past behaviour encourage spillover via influencing three self-perceptions: (i) self-efficacy (perceptions of ability and confidence that one can engage in behaviours that protect the environment); (ii) environmental self-identity (the extent a person seems themselves as being someone who cares about the environment), and (iii) contribution ethic (believing one has ‘done their bit’ in engaging in behaviours that protect the environment).Chapter 2 provides initial evidence (Study 2.1 N = 473) that self-efficacy mediates the relationship between easy, water-related environmental behaviours and more difficult water-related behaviours. Study 2.2 (N = 165) used longitudinal data to extend these findings by demonstrating a serial mediational relationship—where performing easy, water-related behaviours is positively associated with self-efficacy, which is positively associated with intentions to perform more difficult water-related behaviours, and in turn, self-reported adoption of more difficult water-related behaviour (i.e., installing water efficient appliances).Using experimental methods, the remaining empirical chapters (Chapters 3, 4, & 5) focus on how past behaviours influence self-perceptions to elicit spillover to other behaviours. Chapter 3 used a student sample (N = 274) to demonstrate that reminders of a range of everyday environmental behaviours may promote spillover, but only in individuals who performed many of those behaviours in the past. The reminder of many past environmental behaviours (relative to a few behaviours) triggered self-identity and contribution ethic, which was associated with increased intentions for public-sphere, environmental behaviours.Chapter 4 explored whether a single, frequently-occurring behaviour (e.g., putting rubbish in a bin) that is paired with statements designed to trigger self-perceptions (referred to as perception-enhancing information) and information emphasising the importance of acting environmentally, could trigger spillover. In a controlled study using an American community sample (N = 344), providing a message that contained information about past engagement in a single, frequently-occurring behaviour and a message containing perception-enhancing information, triggered self-efficacy, but did not influence subsequent engagement in public-sphere behaviours. These findings highlight the difficulties of promoting spillover in those who perform few environmental behaviours.Chapter 5 sought to further understand whether behavioural prompts could be augmented by using recently-performed behaviours or other information. In a controlled study using an Australian community sample (N = 953), it was found that a single behavioural prompt was able to elicit spillover to public-sphere behaviours, when paired with either information about the recency of past behaviour or the same perception-enhancing information from Chapter 4. A prompt about many past behaviours was able to elicit spillover, independently of the number of behaviours previously performed, when paired with perception-enhancing information. In both cases, spillover was mediated by self-identity and behavioural intentions.The thesis concludes with a broad discussion summarising the findings, and also the implications and challenges of the research. In summary, this thesis presents a series of novel studies examining how different dimensions of past behaviours can be made salient as a means for fostering self-perceptions and spillover. While breadth of behaviours may elicit spillover in individuals who perform many behaviours, to promote spillover in individuals who may perform fewer behaviours, behavioural reminders require pairing with perceptionenhancing information. A key contribution of this thesis relates to studying multiple selfperceptions, which is important because of their potential inter-relationships. Of the three self-perceptions investigated, environmental self-identity was the most robust mediator of behavioural spillover.