Writing about Islam in a single volume is a daunting task, but it is one that I happily took on because of my longstanding desire to help more people in the English-speaking world understand and appreciate this religion. Islam is not only a world religion, claiming about a fifth of the world’s population, it is also a system of culture and politics. Muslims are found in most countries of the world, speaking most of the world’s languages. There is no central authority that can speak for all Muslims, and there is no single way to be a Muslim. It is, like the other great religions of the world, diverse, dynamic, and difficult to define in only a few words, terms, and entries.