Periodization is a concept, not a model. It is a systematic attempt to gain control of the adaptive response to training in preparation for competition. There is very little “hard science” to substantiate periodization. It is mostly based on scientific inferences rather than hard scientific evidence. On the other hand there is an immense body of coaching evidence going back into the early twentieth century that underscores the key elements of what eventually became known as periodization. These key elements are: