Given that much of the world’s current environmental damage can be attributed to unsustainable consumption behaviour, green consumerism can be employed as a means to decrease the severity of these environmental issues. However, although many individual-level psychological variables have emerged as related to green consumerism, there has been little focus within the current literature on how group-level variables contribute to green consumerism, including such variables as social identity and group stereotypes. Thus, the central aim of the present thesis was to explore how environmentalist social identity and environmentalist stereotypes each contributed to green consumerism amongst individuals who were not members of environmental organisations (i.e., ‘environmentalist outgroup members’ or ‘non-environmentalists’). Drawing upon a review of the relevant literature and the social identity approach (Chapters 2 and 3), four research questions were developed. These were: 1) what do environmentalists and non-environmentalists perceive to be involved in green consumerism; 2) what is the content of the stereotypes that non-environmentalists hold of the superordinate environmentalist social category; 3) is environmentalist social identity predictive of green consumerism amongst non-environmentalists; and 4) does environmentalist social identity mediate the relationship between outgroup stereotypes of environmentalists and green consumerism for non-environmentalists. As these research questions focused on broadly exploring the relationship between social identity processes and green consumerism in non-environmentalists, the present thesis employed a mixed methods research approach, and primarily used samples of individuals who did not belong to environmental organisations, rather than self-identified environmentalists. The empirical work in this present thesis consisted of four studies, each exploring one of the research questions. Chapter 4 presents Study 1 (N = 28), where a qualitative analysis of one-on-one interviews demonstrated that self-identified environmentalists and non-environmentalists alike perceived green consumerism to be both an individual and collective pro-environmental behaviour that involved a number of environmentally friendly consumption actions. Study 1 also presented potential reasons as to why some individuals of the general public may fail to identify with the environmentalist social category, with participants suggesting that those who did not consistently engage in pro-environmental behaviours, or who were not ‘active’ enough, could not be labelled an environmentalist. Furthermore, participants also suggested that there were too many negative perceptions surrounding the environmentalist social category, which would decrease the likelihood of some identifying as environmentalists. In Chapter 5, Study 2 (N = 89), an inductive, open-ended qualitative survey was employed to further explore how the environmentalist social category was broadly perceived by non-environmentalists. This study demonstrated that non-environmentalists held a stereotype of the environmentalist social category that incorporated both positive traits (caring, informed, and dedicated) and negative traits (pushy, stubborn, and arrogant). Furthermore, non-environmentalists saw all environmentalists as protective and concerned for the natural environment, yet also acknowledged that environmentalists would not engage in all the same pro-environmental behaviours. Study 3 (N = 275), presented in Chapter 6, used a quantitative survey to investigate whether environmentalist social identity, as well as both individualistic pro environmental behaviours and collective environmental actions, were predictive of green consumerism in non-environmentalists. A path analysis demonstrated that environmentalist social identity, willingness to sacrifice, environmental activism, and political consumerism were all predictive of green consumerism, with environmentalist social identity found to be the strongest predictor of the four. Willingness to sacrifice, environmental activism and political consumerism were also found to partially mediate the relationship between environmentalist social identity and green consumerism, suggesting that environmentalist social identification was the mechanism that underpinned engagement in numerous pro-environmental behaviours including green consumerism. Chapter 7 presents Study 4 (N = 248) which employed a quantitative survey to investigate whether environmentalist social identity mediated the relationship between the endorsement of environmentalist stereotypes and green consumerism for non environmentalists. Findings of this study revealed that perceptions of positive and negative stereotypical traits of environmentalists directly contributed to non environemtnalists strength of environmentalist social identity. A mediation analysis also demonstrated that environmentalist stereotypes exerted an indirect effect on green consumerism through environmentalist social identity. Non-environmentalists who endorsed positive traits as stereotypical of environmentalists were more likely to identify as an environmentalist, and subsequently engage in higher rates of green consumerism. By contrast, non-environmentalists who endorsed negative traits as stereotypical of environmentalists were less likely to identify as an environmentalist, and engage in lower rates of green consumerism. The thesis contributes to the research literature by confirming that environmentalist social identity and environmentalist stereotypes do indeed play a role in green consumerism, at least for those individuals in the general public who are not already members of environmental organisations (i.e., non-environmentalists). The findings of these four studies also contribute to broader discussions concerning how researchers, policy makers, and environmentalists alike can encourage non environmentalists to engage in pro-environmental behaviour, by demonstrating the importance of social identity processes for these behaviours. Chapter 8 therefore concludes the thesis by not only summarising and integrating its findings, but by making suggestions based on these empirical findings, and on the social identity approach, for ways to encourage non-environmentalists to identify with the environmentalist social category and to engage in green consumerism.