Sectarian divisions within the Islamic world have long been misunderstood and misconstrued by the media and the general public. In this book, Adam R. Gaiser offers an accessible introduction to the main Muslim sects and schools, returning to the roots of the sectarian divide in the Medieval period. Beginning with the death of Muhammed and the ensuing debate over who would succeed him, Gaiser outlines how the umma (Muslim community) came to be divided. He traces the history of the main Muslim sects and schools - the Sunnis, Shi'ites, Kharijites, Mu'tazila and Murji'a - and shows how they emerged, developed, and diverged from one another. Exploring how medieval Muslims understood the idea of 'sect', Gaiser challenges readers to consider the usefulness and scope of the concept of 'sectarianism' in this historical context. Providing an overview of the main Muslim sects while problematising the assumptions of previous scholarship, this is a valuable resource for both new and experienced readers of Islamic history.