Expression of recombinant proteins as inclusion bodies in bacteria is one of the most efficient ways to produce cloned proteins, as long as the inclusion body protein can be successfully refolded. Aggregation is the leading cause of decreased refolding yields. Developments during the past year have advanced our understanding of the mechanism of aggregation in in vitro protein folding. New additives to prevent aggregation have been added to a growing list. A wealth of literature on the role of chaperones and foldases in in vivo protein folding has triggered the development of new additives and processes that mimic chaperone activity in vitro.