Interactions of proteins with other macromolecules or small molecules play important roles in most biological processes. Often, such interactions are weak and transient, and the complexes do not easily crystallize. NMR spectroscopy has the unique ability to retrieve information about these interactions and is increasingly used. Recent methodological developments have helped characterize weak protein interactions, and have in particular been applied to the study of proteins that are mostly unfolded alone but form well-defined complexes upon interaction. In addition, NMR methods have been applied to the identification and characterization of small chemicals that inhibit protein function, a primary objective of rational drug design.