This essay summarizes and integrates my experiences and observations-starting in the middle 1970s-as an athlete, scientist interested in human performance, biomedical researcher, and "expert," who sometimes advises athletes, coaches, and sports policy-makers. In this context, my focus has been primarily on endurance sports and five concepts underpin what I have learned over the last 50 years. (1) The "competitive significance principle" whereby athletes, coaches, and policy-makers are frequently interested in performance improvements of 1% or less. This is especially true at the elite level. (2) Science frequently explains what coaches and athletes have already discovered in the field. (3) Many science-based performance improvements have reduced the energy cost of moving a given speed. (4) Varied and eclectic training programs have been used by elite athletes with similar performance outcomes. (5) A few things that are actionable matter. Coaches, athletes, and administrators should focus their efforts on what is actionable.