The program relax is a software package designed for the study of molecular dynamics through the analysis of experimental NMR data. Organic molecules, proteins, RNA, DNA, sugars, and other biomolecules are all supported. It was originally written for the model-free analysis of protein dynamics, though its scope has been significantly expanded. relax is a community driven project created by NMR spectroscopists for NMR spectroscopists. It supports a diverse range of analyses: Model-free analysis - the Lipari and Szabo model-free analysis of NMR relaxation data. R1 and R2 - the exponential curve fitting for the calculation of the Rx NMR relaxation rates. NOE - the calculation of the steady-state NOE NMR relaxation data. Consistency testing of multiple field NMR relaxation data. RSDM - Reduced Spectral Density Mapping. Frame order and N-state model - study of domain motions via the N-state model and frame order dynamics theories using anisotropic NMR parameters such as RDCs and PCSs. Stereochemistry investigations.