The development (and in some part, the functioning) of T lymphocytes occurs providing their contact with epithelial cells. The interaction between T lymphocytes and epithelium is displayed especially clear in the thymus. Chemotactic signals, the source of which are thymic epithelial cells (TEC), play important role in thymus seeding with precursor cells. It has been found that pairs of adhesion molecules are involved in the formation of contacts between thymocytes and TEC: CD2-CD58, beta1-integrin VLA-4-VCAM-1, beta2-integrin LFA-1-ICAM-1 (the first are given molecules of thymocytes, the second - TEC). T cell differentiation events in the thymus can be united into two groups: 1) transition from precursor cells, deprived receptors for antigen (TCR), to T cells with ability to recognize antigen, and 2) divergence of single trunk of TCR-alpha/beta(+) T cells on two major subpopulations (CD4(+) and CD8(+)) with acquisition of functional maturity by them. The basis of thymocyte interaction with TEC in positive selection of thymocyte clones is recognition them by receptors of complexes of autologous MHC molecules with autologous peptides. Thymocytes are subjected to apoptosis at the all stages of their development provided the absence of contact support with stromal cells and growth factors. The ability of TEC to induce thymocyte apoptosis is revealed in their joint culture. The data presented give clear evidence about interdependency of development and functioning of thymocytes and TEC, as well as about symbiotic character of their interrelations.