Follicular unit transplantation is a method of hair restoration surgery where hair is transplanted exclusively in its naturally occurring, individual follicular units. The evolution and rational for follicular unit transplantation will be discussed, as well as the logic for the various techniques used in its implementation. Specifically, the logic for single strip harvesting, stereo-microscopic dissection, automated graft insertion, and large transplant sessions will be reviewed. The central role of the follicular unit constant in the surgical planning will also be discussed.