Bionanocomposites based on the association between biological polymers and inorganic colloids are an emerging class of materials, with main applications in biotechnology and biomedicine. They combine the chemical diversity, hierarchical structure, and biocompatibility of natural biomacromolecules with the robustness and functionality of mineral phases. In particular, biopolymer hydrogels can act as templates and/or host matrices for nanoparticles to design bionanocomposites with tailored optical, conductive, magnetic, mechanical, and bioactive properties. This review presents the key concepts on which such materials are currently designed, in terms of chemistry and physics. Specific examples are provided to illustrate the importance of the bio-organic/inorganic interface on the final properties of the composite structures. It is finally suggested that bionanocomposites have a major role to play for the development of green materials and bio-responsive devices. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012