Every day, people around the world struggle and strive as they challenge injustice and inequality through collective action. They petition, they protest, and they call for changes in public policy to secure rights and status for themselves and for the groups to which they belong. From revolutions in the Arab Spring, to reactionary demonstrations by the US Tea Party, to worldwide protests of Russia's annexing of Crimea, the twenty-first century has seen people on the march.Most existing social psychological research into collective action has focused on understanding the motives for collective action by people who are directly involved in a context of group conflict or inequality ("insiders"). But will external observers ("outsiders") also take collective action when they encounter an external conflict? Who will they support in that conflict? Do their motives for intervening through collective action differ from those of insiders? In this thesis I aim to answer these questions.Chapter 1 establishes the need to consider outsiders as potential collective actors. I review the existing collective action literature and discuss its historical and current focus on insiders. I then introduce outsiders, hypothesise how they may differ from insiders, and argue for the importance of understanding how outsiders respond when they encounter group conflict.Chapter 2 includes two studies that sought to determine (1) whether outsiders are willing to take collective action on behalf of an external conflict, and (2) whether outsiders and insiders' motives for action differ. I used the Social Identity Model of Collective Action (SIMCA; van Zomeren, Postmes, & Spears, 2008) to measure appraisals of income inequality in the United States by residents of both the US (insiders) and India (outsiders). SIMCA is a well-established framework for explaining insider collective action, and specifies that group identification, perceived injustice, and perceived group efficacy are each unique motives for collective action. In both studies, outsiders were willing to take collective action. In addition, both outsiders' and insiders' motives for action could be captured using the SIMCA framework. Outsiders perceived the conflict as less self-relevant than insiders, and reported less identification with the disadvantaged group than insiders; a personal connection with the group conflict or inequality is thus necessary for outsiders to take collective action. To understand and predict outsider collective action, I must then investigate how outsiders come to see an instance of group conflict as self-relevant.Chapter 3 explores how ideological orientations shape outsiders' appraisals of a group conflict, and whether outsiders may also choose to support a relatively advantaged group in conflict.In two studies, I investigated how Social Dominance Orientation (SDO; Pratto, Sidanius, Stallworth, & Malle, 1994) and Right-Wing Authoritarianism (RWA;Altemeyer, 1996) shape US residents' appraisals of separate conflicts between disadvantaged citizens an...