The characteristics of biological systems of adaptation are developed from the principle that the manifestations of life are modified by and must conform with their environment in order to enable organismic persistence. The two roles of the environment, termed "modifying" and "adaptive", give rise to the distinction of three elementary and sequentially coupled subsystems characterizing the adaptational system: the modifying system, comparator system, and state regulation system. The third system determines which state alterations are required for adaptation of manifestations (responses) to the demands of the adaptive environment. Adaptational valuation is introduced as a measure of adaptedness of response to adaptive environment. The dynamical aspect of adaptation is shown to be completely described by the timing of feedback. Discrete and continuous forms of adaptation can thus be treated on the same conceptional basis. All steps of the generic system formulation are illustrated with the help of a simple model with additive effects and linear state regulation. The system representation is used to demonstrate how basic intuitive conceptions, such as adaptational lag or adaptational capacity, can be made amenable to precise analysis. Another demonstration concerns recognition of adaptational clues resulting from the distinction between the two environmental classes, modifying and adaptive. Among these clues are challenges of low correlation between the two classes to adaptational systems and the specification of questions of the evolution of phenotypic plasticity.