This chapter starts with the potential of linear regular waves on deep water, proceeds to nonlinear Stokes waves and further to computing periodical long-crested steep waves of arbitrary order for deep and shallow water. Then methods to simulate a natural (irregular) seaway are discussed, using both linear and second-order superposition of regular waves; a higher-order method is only cited. Typical wind wave spectra are described, and diagrams and formulae are given to approximate their parameters from wind speed, fetch and duration. Finally, an example of a scatter table (probability distribution of significant wave period and height) taken from the a modern “wave atlas” is given.