NMR spectroscopy is a very sensitive technique to study the conformation and dynamics of non-native states of proteins. However, the experimental observables have to be interpreted in light of models describing the conformational averaging, in which the number of participating conformational states and the rate of their interconversion are the two most important parameters. Such models range from theoretical predictions of the properties of the polypeptide chain to extensive molecular dynamics simulations. They are being developed to aid the interpretation of NMR parameters such as chemical shifts d, spin-spin coupling constants J, homonuclear NOE data (NOEs), residual dipolar couplings (RDCs) and heteronuclear relaxation properties such as relaxation rates (¹⁵N-R₁, ¹⁵N-R₂) and heteronuclear NOEs ({¹H}-¹⁵N-NOE). We will demonstrate for model proteins, for homopolymeric peptides and for the human prion protein, how NMR can be applied to characterize such diverse states as the unfolded state, the molten globule state and the fibrillic state.