This chapter reviews the types of defects and damage found in composite structures, their failure modes and mechanisms, and a general representation of the defect. The basic description of a composite structure must be restated since this is very important for the understanding of defects and damage in composites. Composite stricture is a heterogenous material in that the fibre and resin system are independent materials. Defects and damage occur to composite components and structures during three fundamental life phases for the composite. These three fundamental phases are: constituent materials, prepreg and weave processing, composite component manufacture and fabrication, and in-service use. The size of a defect or damage in composite components and structures has significant bearing on the strength or performance criticality of the component. Fibre will fracture at much higher loads than the matrix material when under in-plane loads, particularly tensile loads.