Understanding the process of protein folding, during which a disordered polypeptide chain is converted into a compact well-defined structure, is one of the major challenges of modern structural biology. In this article we discuss how a combination of physical techniques can provide a structural description of the events which occur during the folding of a protein. First, we discuss how the rapid kinetic events which take place during in vitro folding can be monitored and deciphered in structural terms. Then we consider how more detailed structural descriptions of intermediates may be obtained from NMR studies of stable, partly folded states. Finally, we discuss how these experimental strategies may be extended to relate the findings of in vitro studies to the events occurring during folding in vivo. The approaches will be illustrated using results primarily from our own studies of the c-type lysozymes and the homologous <latex>$\alpha $</latex>-lactalbumins. The conclusions from these studies are also related to those from other systems to highlight their unifying features. On the basis of these results we identify some of the determinants of the events in folding and we speculate on the importance of these in driving folding molecules to their native states.