The relationship between VO2 and velocity of running (running economy) has been rather casually dealt with until very recently, and there still remains considerable disagreement as to the importance of this variable. Various factors which have been shown, or appear, to affect running economy include environment (temperature, altitude, running surface), fatigue, age, weight, state of fitness, and inherent differences. That differences between individuals and within individuals can and do exist seems clear; the questions which need to be addressed in future research are: (1) What type of training is most effective in bringing about changes in running economy? and (2) How much change in economy can be expected with optimum training? Furthermore, it is suggested that running economy be investigated as an entity, so that changes that may take place with time or training can be more accurately related to their cause.