Reiki (pronounced "Ray-key") was developed by a man called Mikao Usui in the early 1900's in Japan, He created a simple treatment system of using "universal" energy to promote physical and emotional wellbeing.
Reiki is now one of the most popular complementary therapies in the West. Anyone can benefit from Reiki, and it is practised by people from all walks of life. You do not need to have any prior experience of receiving Reiki energy to benefit from it. The effects of Reiki build up cumulatively, so while a one-off Reiki treatment will do something beneficial for you, having a course of 4 treatments is generally the best way to help to produce longer-term beneficial changes for you on many levels.
A Reiki practitioner will not diagnose or recommend any form of treatment; it is not a substitute for medical treatment. If you have any symptoms of illness you should see your doctor. Reiki is, however, a safe, holistic therapy that will complement any form of medical treatment.