The mechanism of the threshold of sediment motion under wave action has been investigated by examining the dynamic forces acting on the sediment particle. The force analysis has led to a range of nondimensional parameters of clear physical significance, which are then applied to the analysis of a range of existing experimental data. The introduction of some new nondimensional parameters has resulted in significant improvement in the definition of the threshold of motion condition. Several empirical equations involving different nondimensional parameters have been derived, providing a mathematical description of the threshold condition of the sediment motion. The accuracy of these empirical equations is examined in the context of scientific and engineering applications and a simple equation is recommended for its accuracy and ease of use.