I highlight the importance of social relationships in shaping religious tolerance and political behavior in Muslim-majority countries and the United States. Pertaining to religious tolerance, I propose an explanation for why scholars have found a lower level of religious tolerance in Muslim than in non-Muslim countries. Contrary to the mainstream explanations that focus on theological, cultural, institutional, and socio-economic variables, I argue that Muslim countries' lower tolerance is driven by lower levels of interfaith friendships of Muslims in the countries. Analyzing a survey of more than 15,000 Muslims in 17 Muslim countries, I show that interfaith friendships are related to higher tolerance among Muslims and that Muslims in Muslim-majority countries indeed have lower levels of interfaith friendships than Catholics in Catholic-majority Latin American countries (Chapter 2). I also provide experimental evidence for the relationship by analyzing an original survey experiment that I conducted in the world's largest Muslim-majority country Indonesia (Chapter 3).In the next chapters I demonstrate that social relationships also influence Americans' tolerance toward Muslims and political behavior in general. Analyzing the 2008-2009 ANES Panel I show that individuals embedded in politically diverse networks are less likely to believe in the conspiracy that Obama is Muslim (Chapter 4). This finding complements previous works on the topic that have exclusively focused on political and psychological predispositions. Lastly, I relate social relationships to one of the most hotly debated issues in American politics: polarization (Chapter 5). I show that a mistaken assumption that partisan bridging (relationships with politically different others) and partisan bonding (relationships with politically similar others) are inversely correlated led previous work to associate partisan bridging with positive outparty attitudes and lower political engagement. Employing a novel measurement of bridging and bonding, I demonstrate that bridging and bonding are only weakly correlated and have independent effects on political behavior. Partisan bridging, in particular, offers a remedy for a polarized public as it both improves attitudes toward the outparty and encourages participation among individuals.The contributions of my social relationships-based approach to tolerance extend beyond the academic world by highlighting avenues for policy interventions. The power of social relationships means that our efforts to promote democracy and tolerance in the Muslim world and beyond must not be limited to promoting moderate religious views or institutional improvements but also must include strategies to enhance social integrations between various social groups.