Nuclear magnetic resonance spectroscopy can be used to study the structure of molecules, the interaction of various molecules, and the composition of mixtures of biological origin. Molecule sizes can range from small organic molecules or metabolites, to mid-sized peptides or natural products, or up to proteins of several tens of kDa in molecular weight. NMR complements other structural techniques such as X-ray crystallography. The advantage of NMR is the ability to study molecules in solution or even in biological fluids.