Protein folding is perhaps the most fundamental process associated with the generation of functional structures in biology. There has been considerable progress in the last few years in understanding the underlying principles that govern this highly complex process. Central to much of this progress has been the development of ideas as to the nature of the energy surface or landscape for a folding reaction. These ideas have arisen from a combination of theoretical analysis and experimental investigation. Of particular importance in the latter has been the concerted application of a wide range of experimental techniques, each able to describe aspects of the structural changes taking place during the folding process. NMR spectroscopy and protein engineering have both been key methods in this approach because of their ability to provide structural and dynamical information at the level of individual residues. Recently, new approaches have been devised that combine experimental data directly with simulation techniques to define the structures of key species on the folding surface.