With over one billion adherents worldwide and 15% of the world’s population, Hinduism is the fourth largest, and among the oldest, of the major world religions, with important political aspects that reverberate well beyond South Asia. Yet it is perhaps the least studied of the major world religions. Hinduism is also one of the most geographically concentrated religions of the world. The majority of Hindus are concentrated in two South Asian countries, Nepal and India, where Hindus constitute 80% or more of the population. Small but politically influential diasporic communities of Hindus are found throughout Europe, Africa, the Caribbean, the United States, and Canada. Key characteristics of Hinduism that set it apart from Abrahamic religions (Judaism, Christianity, and Islam), especially politically, include its polytheistic nature and lack of one single authoritative text; the tremendous variation in its practice across locality and caste; and its frequently informal practice beyond the confines of official institutions such as temples. Hinduism has been compatible with a range of regime types over time in India and Nepal, including empire, monarchy, and democracy. Both India and Nepal are officially secular countries, but the status of secularism in both countries is contested by the forces of Hindu nationalism, a movement that seeks to institutionalize the political, social, and cultural predominance of Hinduism. Religious conversion is expressly prohibited in Nepal; in India, it is increasingly under legislative attack. The politics of caste are an important political aspect of Hinduism in both India and Nepal. While politics in both countries remain dominated by upper castes, important lower-caste political mobilization has appeared in India, but has yet to take hold in Nepal. A better understanding of Hinduism’s political aspects has enormous potential to enhance knowledge of religion and politics more broadly.