The basis of ship powering prediction from model experiments is presented together with the different methods in use for extrapolation of ship powering estimates. The methodology used has been to refer back to Froude's Law of Comparison and describe the three tests in use for powering prediction (resistance, propeller open water, and propulsion tests), the use of a form factor and the difficulties of its determination, the contributors to uncertainty in the extrapolation process, and points of attention to watch for in the powering prediction process. The presentation concludes by emphasizing how uncertainty can be reduced and the accuracy of the powering prediction process can be improved.