Theoretical and experimental studies of the past decade on the application of thermal neutrons for studying the properties of biological macromolecules are reviewed. The physical bases are presented for the application of thermal neutrons for studying biological macromolecules. The fundamental approaches to structural problems based on a change in the isotopic composition of the solvent are discussed in detail. Results are presented of the experimental studies with solutions and monocrystals of macromolecules. The fundamental trends in further development of these studies are discussed.