Reiki is a system of natural healing that was founded by Dr. Reiki Usui Mikao hence it is also called Usui Reiki after it's founder. Dr. Usui or Usui Sensei as he is called was a staunch Buddhist who studied esoteric ancient Tibetan and Sanskrit scrolls and meditated on Mount Kurama on the outskirts of Kyoto, Japan. Dr. Usui was also a member of the Rei Jyutu Ka, a metaphysical group dedicated to developing psychic abilities and performing esoteric practices that were looked down upon by traditional monasteries. He was said to have achieved enlightenment there and he synthesized the teachings of Japanese Esoteric Buddhism with Indian and Chinese medicines to form his own brand of healing in which the Universal Healing Light energy was channeled and directed to perform healing. Thus Reiki does not work on merely the physical body but on the subtle body and at the soul level to bring about change. Western medicine treated the physical manifestation of a problem but Reiki treated the problem by correcting the energies in the body or "Ki" and aligning it with the Universal energy to completely cure a disease that were caused by such energy imbalances in the mind, body or spirit.
Soon Dr. Soon Usui's healing method became very popular in Japan and he traveled throughout the country to teach his skill and perform Reiki Empowerments. Dr. Usui also set up a Reiki Association called the The Usui Reiki Ryoho Gakkai where the word Gakkai meant Association and this was the first formal Reiki association and even today it maintains the traditional teachings of Dr. Usui Mikao. He was the first president of this organization and after his death the presidency was given to his best friend, Mr. Ushida.
However this was a time when the West was still unaware of this method healing. Though Usui Reiki or Usui Reiki Ryoho as it was called was the original system of Reiki, over the years it has traveled to the West where practitioners have merged this system with other esoteric healing practices to form many other branches of Reiki which are practiced today. Usui Reiki was bought to the West by a lady called Mrs. Hawayo Takata who lived in Hawaii. She was visiting her realtives in Japan when she came upon this form of healing by a student of Dr. Usui's namely Dr. Hayashi. She underwent treatment everyday for four months for some serious ailments she was suffering. After four months not only was she completely cures but was very impressed by this form of healing. She learnt this form of treatment from him and soon after an Attunement from him in 1938 she began practicing this when she returned to America. She initiated twenty-two Reiki Masters in America and this list has continued to grow and branch out and continuously evolve like a chain reaction.
Dr. Usui wanted to create a spiritual form of healing that was without the denomination of religion or any other boundary. Reiki can be practiced as a way of life without disturbing any religious practices.