The longitudinal (T1) and transverse (T2) hydrogen (1H) nuclear magnetic resonance (NMR) relaxation times of normal human and animal tissue in the frequency range 1–100 MHz are compiled and reviewed as a function of tissue type, NMR frequency, temperature, species,in vivoversusin vitrostatus, time after excision, and age. The dominant observed factors affectingT1are tissue type and NMR frequency (ν). All tissue frequency dispersions can be fitted to the simple expressionT1=AνBin the range 1–100 MHz, withAandBtissue‐dependent constants. This equation provides as good or better fit to the data as previous more complex formulas.T2is found to be multicomponent, essentially independent of NMR frequency, and dependent mainly on tissue type. Mean and raw values ofT1andT2for each tissue are tabulated and/or plotted versus frequency and the fitting parametersA,Band the standard deviations determined to establish the normal range of relaxation times applicable to NMR imaging. The mechanisms for tissue NMR relaxation are reviewed with reference to the fast exchange two state (FETS) model of water in biological systems, and an overview of the dynamic state of water and macromolecular hydrogen compatible with the frequency, temperature, and multicomponent data is postulated. This suggests that1H tissueT1is determined predominantly by intermolecular (possibly rotational) interactions between macromolecules and a single bound hydration layer, and theT2is governed mainly by exchange diffusion of water between the bound layer and a free water phase. Deficiencies in measurement techniques are identified as major sources of data irreproducibility.