Since the Symposium on the Behavior of Ships in a Seaway (Wageningen, 1957), many papers have been published on the theory of ship motions.The present paper is a survey, collation, and evaluation of those contributions which have led toward a rational theory for predicting ship motions.During this period, evidence has accumulated which demonstrates the validity of the superposition principle for ship motions in a seaway. This concept was stated as hypothesis eleven years ago by St. Denis and Pierson (and also sixty years ago by R. E. Froude); its validity may now be considered as proven, beyond the fondest hopes of earlier investigators.With this principle established, attention once again returns to the prediction of motions in small-amplitude regular waves. The best practical approach to making ship motions predictions is probably still through use of strip theory. However, the two-dimensions assumptions 'A more precise definition is that p[aj