Our knowledge concerning stars of the T Tauri and UV Ceti types has been considerably enlarged during recent years, owing to the accumulation of a large number of facts. As a result, we may now make certain suggestions regarding the causes of the non-stability of these stars, based on observational data. Such an approach to the solution of the problem is altogether different from that applied heretofore in attempts to develop a theory of certain types of variable stars. The method of construction of models has usually been applied in the past. We do not exclude, of course, the possibility of solving a problem on the basis of some theoretical model. However, the physical phenomena observed in the T Tauri and UV Ceti stars are so unusual that it is first of all necessary to define the nature of these phenomena and their mutual interrelation. Only then will detailed mathematical construction of a definite model be possible.