Synopsis An attempt has been made to provide an in-depth study of 1.2 billion Muslims of the world—who and where they are and how they live. It seeks to discover what they have in common, what distinguishes them from each other and what they feel about themselves. Here, we have identified 750 Muslims tribes, castes and communities. Of these, 246 Muslim groups are Indian—90 are in the northern, 73 in the western, 35 in the eastern, 20 in the central, 20 in the southern and 8 in the north-eastern regions of the country. Besides them, there are some groups who have undergone changes of their identity altogether. The entries of this Encyclopaedia show that Muslims have much in common, they share an universal trans-ethnic religion-Islam, the second largest religion after Christianity. They worship the same God, recognize more or less the same religious laws, but observe many of the same rituals and share distinguishable culture traits derived from their Islamic heritage.