Biological materials, such as wood, grasses, protein fibres, bone, sea shells or glass sponges are generally composites of different types of polymers as well as mineral. These materials are able to adapt to the mechanical requirements of the environment by growing and assembling structural components in a hierarchical fashion. This implies the existence of various types of interfaces at all levels of hierarchy. From a mechanical viewpoint, interfaces may be considered as defects but, in many natural materials, interface structures emerged which improve rather than deteriorate the overall mechanical properties of the composite. Bone, for example, consists in about equal amounts of a collagen-rich matrix and calcium-phosphate nano-particles. These components are joined in a complex hierarchy of fibres and lamellar structures to a material with exceptional fracture resistance. Similarly, tendon collagen consists of an assembly of fibrils which partly deform by shearing the interface between them. An example for self-healing properties due to molecular-scale interfaces is the byssus fibre used by mussels to attach to rocks. These fibres combine large deformation with stiffness and abrasion resistance. Finally, plant cell walls generate internal stresses and even complex movements upon changes of environmental humidity. This force generation and actuation capabilities are based on water swelling of hemicellulose-rich interfaces between cellulose microfibrils arranged in complex architectures. Unravelling the structural principles of these unexpected material properties may indicate ways towards new types of composite materials with adaptive capabilities.