We give in this paper an exposition of the fundamental theory underlying the theory of reflection and refraction seismic waves. It was thought useful to include material not strictly new along with the new material, because the sources are not easily obtained by many readers of this magazine. Knott’s energy equation is given a simple interpretation. Conditions at the interface are discussed together with the limitations of these conditions, as conditions by which our problem is to be solved. It appears that the energy flow must be considered as well as the interface conditions.