A new method is presented for computing the added resistance of a ship advancing at constant mean speed and heading in regular waves. It is shown that the added resistance, even though it is a second-order quantity, can be expressed as a product of first-order terms that are all computed by computer programs presently in use for predicting the linear heave and pitch motions. Comparisons between the present theory and other theories, as well as experimental results, are presented for head waves. It is found that the discrepancies in the experimental results can be as large as the differences between different theories. A difference of 30% between experimental values for the maximum added resistances seems to be typical. It is concluded that the present theory predicts the added resistance with better accuracy for a wider range of ships forms and speeds than any other existing theory. It is also shown how the mean added resistance in a short-crested seaway can easily be obtained from the regularwave results.