- 2015 Elsevier Ltd All rights reserved. This chapter presents the fundamental considerations in treating durability of composite materials with fibrous reinforcement. General concepts underlying irreversibility in stress-strain behavior are addressed and the associated dissipative mechanisms are discussed. Particular focus is placed on fatigue, for which a conceptual framework for interpretation of the roles of fibers, matrix, and fiber/matrix interface, introduced by the author in a 1981 paper, is described. The extension and variation of the baseline framework, called the fatigue life diagram, to general cases of loading and fiber architecture are treated. Finally, guidelines are offered for modeling changes in material response and consequent loss of functionality under different fatigue conditions.