There are three schools of thought in the study of sediment transport based on the concept that rate of energy dissipation of flowing water should be related to the rate of sediment transport or concentration. They are the unit stream power by Yang, the stream power by Bagnold, and the gravitational theory by Velikanov. Theoretical analyses are made to compare the validity of these theories. Laboratory and river data are used to test the assumptions used in the derivation of sediment transport equations based on these theories.