This groundbreaking work explores the powerful role of communities in mathematics. It introduces readers to twenty-six different mathematical communities and addresses important questions about how they form, how they thrive, and how they advance individuals and the group as a whole. The chapters celebrate how diversity and sameness bind colleagues together, showing how geography, gender, or graph theory can create spaces for colleagues to establish connections in the discipline. They celebrate outcomes measured by mathematical results and by increased interest in studying mathematics. They highlight the value of relationships with peers and colleagues at various stages of their careers. Together, these stories offer a guide—rather than a template—for building and sustaining a mathematical community. They call attention to critical strategies of rotating leadership and regular assessment and evaluation of goals and programs, and promote an ongoing awareness of the responsibilities of life that impinge on mathematical creativity and contributions. Whether you are giving thought to starting a group, joining one already in existence, or encouraging a colleague to participate in the broader mathematical community, this book will meet you where you are—and move you beyond. It contains a plethora of ideas to foster a sense of belonging in the exciting discipline of mathematics.