It has only been about 3 years since natural orifice translumenal endoscopic surgery (NOTES) first began to gather the attention of the medical and surgical community at large. The concepts behind NOTES, however, have been under development for almost a decade. It is important to revisit some basic concepts regarding therapeutic flexible endoscopy and in the process understand the fundamental premises on which a sound technology development program for NOTES should be built. This article provides such a review and emphasizes general principles rather than specific embodiments, which are very much at a nascent stage.