The denatured "state" of a protein is a distribution of many different molecular conformations, the averages of which are measured by experiments. The properties of this ensemble depend sensitively on the solution conditions. There is now considerable evidence that even in strong denaturants such as 6M GuHC1 and 9M urea, some structure may remain in protein chains. Under milder or physiological conditions, the denatured states of most proteins appear to be highly compact with extensive secondary structure. Both theoretical and experimental studies suggest that hydrophobic interactions, chain conformational entropies, and electrostatic forces are dominant in determining this structure. The denaturation reaction of many proteins in GuHC1 or urea can be most simply modelled as a two-state transition between the native structure and a relatively compact denatured state, which then undergoes a gradual increase in radius on further addition of denaturant. However, when a protein acquires a large net charge in acids or bases, it can have two stable denatured populations, one compact and the other more highly unfolded. The prediction and elucidation of the structural details of the non-native states of proteins may ultimately prove to be as difficult as predicting the native structures, particularly for D0, the denatured state under physiological conditions. Just as with the native state, the structure of this biologically important denatured state appears to depend on the amino acid sequence. The development of synthetic, peptide and protein fragment models of the denatured state and the recent progress in NMR spectroscopy provide bases for optimism that new insights will be gained into this poorly understood realm of protein biochemistry.