It is believed that reiki originally evolved as a branch of Tibetan Buddhism and that knowledge of its power and how to use it was transmitted from master to disciple. At some point in the intervening centuries, the secrets of reiki were lost. They were rediscovered in the late 19th century by a Japanese minister, Dr. Mikao Usui. He spent 14 years seeking the ability to heal, which he believed could be discovered thiough studying Buddhism, learning Chinese and Sanskrit to help his research. It is claimed that he eventually found the knowledge he sought in an Indian sutra, or sacred text. Then, after a three-week meditation on a mountain top, he had a vision of four symbols that could be used to enable healing energy to be passed to others. The ability to channel the healing power was achieved by attunement to each of these symbols. Before he died in the 1930s, Dr. Usui initiated 16 others into the secret of reiki, teaching them the master attunement.

One of these reiki masters was Dr. Chujiro Hayashi, who undertook to preserve reiki and pass it on. He brought scientific training to bear on the method, noting down the results of healing sessions, codifying the sequence of hand movements in a reiki session, and establishing a reiki clinic in Tokyo.

Treatment by a reiki practitioner is intended to promote physical, emotional, and spiritual well-being. Clients lie fully clothed while the practitioner's hands are placed on specific parts of the body, starting with the head. Some reiki practitioners do not touch the physical body but transmit healing into the surrounding aura. Reiki can be used to heal the self or someone else, and reiki energy can be projected into the future or directed to a distant place. It can also be used on pets or plants.

Anyone can learn reiki because it is not taught in the strict sense of the word, but transferred from teacher to pupil. This transferral is made though a series of "degrees." The first degree usually takes a weekend and four initiations by a reiki master. It does not involve changing religious beliefs. These initiations impart the ability to transmit healing energy to oneself and others. The second degree deepens participants' experience and enables them to give distant healing. The third degree is taken by those wishing to become reiki masters, able to teach others.

The results of treatment can be dramatic, or more gradual, showing themselves in general improvements in health and well-being. Reiki healing energies can have lasting results only if the recipient accepts his or her responsibility in the healing process and takes part in it. Daily self-treatment is regarded as preventive, supporting emotional and spiritual growth.

There are more than 200,000 reiki practitioners and centers in North and South America, Europe, and Australasia. The word "reiki" is a generic term in Japanese and is not exclusive to Dr. Usui's healing method; this is more formally known as Usui Shiki Ryoho or the Usui system of natural healing, and is preserved by the international Reiki Alliance, which has agreed professional standards and a code of ethics.