In this unique compendium, the general principal of adaptation and its application in communications, especially for bit rate adaptation are considered. Two type of bit rate adaptation are treated — signal duration and size constellation. When a channel state is relatively good, size constellation adaptation should be used. With state degradation, signal duration is used. The optimal control algorithms maximizing bit rate are constructed, and quantitative characteristics of efficiency are determined for both types separately as well as when they are used jointly. The characteristics are separated into two classes: potential and real ones. The potential demonstrates the limiting possibilities for the system and real shows how much the efficiency decreases, taking into account the influence of real factors.