Protein aggregation seems to be a common feature of several neurodegenerative diseases and to some extent of physiological aging. It is not always clear why protein aggregation takes place, but a disturbance in the homeostasis between protein synthesis and protein degradation seems to be important. The result is the accumulation of modified proteins, which tend to form high molecular weight aggregates. Such aggregates are also called inclusion bodies, plaques, lipofuscin, ceroid, or 'aggresomes' depending on their location and composition. Such aggregates are not inert metabolic end products, but actively influence the metabolism of cells, in particular proteasomal activity and protein turnover. In this review we focus on the influence of oxidative stress on protein turnover, protein aggregate formation and the various interactions of protein aggregates with the proteasome. Furthermore, the formation and effects of protein aggregates during aging and neurodegeneration will be highlighted.