We would like to introduce bionanoparticles with their unique multifunctional and self-assembling properties. Particularly, protein cages like plant viruses or ferritin but also other well-defined self-assembling protein structural motifs are valuable building blocks with great potential in (bio-) nanotechnology. A steeply increasing number of research works present promising results and applications in biomedicine, diagnostics and analytics as well as nanoelectronics. However, the use of bionanoparticles for hybrid and soft protein-polymer composite materials has not received high attention yet. The article will first introduce the structural principles of well-defined protein complexes and exemplarily describe the structure of a few selected plant viruses and ferritin. Then, the recent progress in chemical or genetically programmed functionalization and the use of the modified bionanoparticles for the production of novel nanostructured (hybrid) materials will be presented. An updated overview of grafting-onto and grafting-from polymerization methods for the modification of proteins and protein complexes will be given as well. The feature closes with some exciting examples in which bio (in-) organic nanoparticles are employed in analytics, for catalysis and biomedical applications. (C) 2010 Elsevier Ltd. All rights reserved.