Transplantation is a life saving process and it has evolved tremendously over the last 300 years. Currently we are able to better understand the immune system and its response which is absolutely responsible for the success of every transplanted organ. This understanding has come about by many a scientist trying to narrow down which cell exactly is responsible for the process we refer to as rejection which is when the body into which the organ may be transplanted does not accept the organ. The actual cells thought responsible have changed over the years and as such are medications used to arrest or slow these cells down have changed. This course is intended to give an overview of how we have evolved in the field of transplantation, not only clinically but scientifically. A description of major landmark studies and operations will be highlighted followed by a review of the immune system and its cells that have been investigated over the last several decades.