The classical in vitro denaturation-renaturation studies by Anson, Anfinsen, Neurath, Pauling and others clearly suggested that the primary structure of proteins determines all higher levels of protein structure. Protein folding in the cell is inaccessible to a detailed analysis of its kinetic mechanism. There are obvious differences: nascent proteins acquire their native structure co- and post-translationally, with half-times in the minutes range, whereas refolding starts from the complete polypeptide chain, with rates varying from seconds to days. In the cell, accessory proteins are involved in regulating the rate of folding and association. Their role can be analysed both in vivo, by mutant studies, or by coexpression together with recombinant model proteins, and in vitro, by folding experiments in the absence and in the presence of `foldases' and molecular chaperones, with the following general results: (i) folding is a sequential process involving native-like structural elements and a `collapsed state' as early intermediates; (ii) the major side-reaction is caused by `kinetic partitioning' between correct folding and wrong aggregation; (iii) rate-determining steps may be assisted by protein disulphide isomerase, peptidyl prolyl-cis-trans-isomerase, and molecular chaperones; and (iv) extrinsic factors, not encoded in the amino acid sequence, may be of crucial importance.