The system of Reiki was developed in the 1900’s by a Japanese man named Usui Mikao. Mikao’s teachings involved a blend of esoteric Buddhism (Mikkyo), ancient Shugendo practices, classical samurai teachings and martial arts know-how. He utilised these traditional approaches in his own inimitable way with the intention to bring about an unmasking, a revelation of what it meant to be human (The Japanese Art of Reiki, Bronwen & Frans Steine).
These earlier teachings were focused toward spiritual development, but later evolved to take on a more structured approach, which incorporated symbols, mantras and attunements, in addition to the precepts and meditations. This original system of Reiki and its development are still being researched today.
Usui Mikao had 21 master students in whom he passed on his teachings. One of these, Hayashi Chujiro (link), is credited with teaching Hawayo Takata (link), who first took Reiki from Japan and introduced it to Hawaii in 1938. From there Reiki spread throughout America and then the western world. Hawayo Takata taught 22 masters before she passed away in 1980. During her time she developed what is commonly referred to as "the western system" of Reiki.
Since Hawayo Takata's passing, Reiki has become widespread, with many variations in methodology and philosophy. This has created a lot of confusion as to the origins and initial teachings underpinning the system, which has led many people to seek answers back in Japan, the birthplace of Reiki. Slowly, researchers are liaising with some of the traditional Japanese Reiki teachers and learning about the system and its founder, Usui Mikao. Greater understandings of the cultural and spiritual influences at the time of its inception are beginning to provide clarity into the system, as it was originally developed.