This chapter aims to explore Muslims’ religion/spirituality and discuss the factors that are important to consider in scientific research and clinical practice at the intersections of positive psychology, religion, and spirituality. The discussion is based on the theological teachings of the Islamic religion, theoretical speculations regarding the links between Islam and positive psychology, and the relevant findings of empirical studies conducted with Muslim samples. The chapter is composed of five sections. The first is an overview of the geographic distribution, demographic composition, and intrafaith diversity of Muslims around the world. The second section reviews key Muslim religious/spiritual beliefs, teachings, practices, and texts. The third section discusses common Muslim understandings of virtues, health, and well-being. The fourth section reviews potential implications for science and practice at the intersections of positive psychology, religion, and spirituality. The final section offers recommendations for guiding research and practice when working with Muslims at these intersections.