Unitarian Universalism (UUism) is a theologically liberal religion characterized by its support for a "free and responsible search for truth and meaning." Unitarian Universalists do not share a creed; rather, they are unified by their shared search for spiritual growth. Unitarian Universalists draw on many different theological sources and have a wide range of beliefs and practices.

Both Unitarianism and Universalism trace their origins to Christian Protestantism and thus Unitarian Universalism has its historical roots in the Christian faith. But by the time they decided to combine their efforts at the continental level, the theological significance of these terms had expanded beyond the traditional Christian understanding. Today’s UUs appreciate and value aspects of other religions ranging from Judaism to Buddhism. Although Unitarian Universalist congregations and fellowships tend to retain some Christian traditions, such as Sunday worship with a sermon and the singing of hymns, they do not necessarily identify themselves as Christians, nor do they necessarily subscribe to Christian beliefs. The extent to which the elements of any particular faith tradition are incorporated into one's personal spiritual practices is a matter of personal choice in keeping with Unitarian Universalism's creedless, non-dogmatic approach to spirituality and faith development.

The Unitarian Universalist Association (UUA), founded in 1961 as a consolidation of the American Unitarian Association and the Universalist Church in America, is headquartered in Boston, Massachusetts, and serves churches mostly in the United States. The Canadian Unitarian Council became an independent body in 2002. The UUA represents more than 1,000 member congregations that collectively include more than 217,000 members. According to the United States Census Bureau 629,000 individuals identified themselves as Unitarian/Universalist in 2001. A more recent survey (2007) performed by the Pew Forum on Religion & Public Life found that 0.3% of U.S. adults or approximately 340,000 individuals identified themselves as Unitarian Universalist. Unitarian Universalists follow a congregational model of church governance, in which power resides at the local level; individual congregations call ministers and make other decisions involving worship, theology and day-to-day church management. The denominational headquarters in Boston in turn provides services for congregations that can more effectively be handled through joint efforts.

A separate organization from the UUA is the International Council of Unitarians and Universalists (ICUU), founded in 1995, which coordinates national Unitarian and Universalist associations of churches throughout the world.