This is a short review of investigations of an applied character, performed at the Department of Histology and Embryology of the Academy. Most of the investigations have been carried out as joint works with clinicians and are aimed to solve some actual tasks of practical medicine. Their main contents have been investigations on reactivity and degeneration of tissues under various experimental and clinical conditions. Some new data have been obtained concerning cellular and ultrastructural mechanisms of the wound process, scientifically grounded influences have been undertaken in order to regulate this process. The material presented has served as a base for elaboration of a number of fundamental principles of histology and first of all some new ideas on histogenesis, cellular-differon organization and reparative regeneration of tissues. Regularities of histo- and organogenesis and intertissue interrelation are the base for understanding regeneration mechanisms of organs and tissues. In what form the reparative regeneration is manifested depends on histogenesis and is specific for each tissue. To know regularities of histogenesis and peculiarities of its manifestation is necessary to understand problems of pathology, since regularities of the normal histogenesis serve as a foundation for understanding the essence of pathological processes.