It is, therefore, necessary to study the magnitude of genotype-environment interactions under the environment where the breeding program is going to be used. The most important traits to be improved are: growth rate, feed efficiency, age at sexual maturation (when producing large fish), disease resistance and meat quality. In rainbow trout, individual and family selection are the most promising selection methods. Since additive genetic variance seems to be present for all economically important traits, selection should always be used in breeding programs, and crossbreeding should be included only when heterosis is large. Since fecundity is high in rainbow trout the breeding program can be concentrated at one or a few breeding centers. The importance of evaluating breeding values of fish under practical farming conditions (test stations) is stressed. Multiplier stations should be developed according to needs. It is concluded that selection is a powerful tool to increase productivity in rainbow trout and that it will increase the rate of domestication.