Extremely important developments occurred in the study of close binaries in recent years. The preceding triennium, 1967–69, witnessed the formulation of the theory of mass transfer between the components of a close binary. Evolutionary calculations using mainly Henyey’s method explained — at least qualitatively — the existence of the semi-detached binaries, and predicted formation of white dwarfs and helium stars in certain close binary systems. In connection with these studies, a number of theoreticians working on stellar evolution contributed to the study of close binaries. In the triennium 1970–72, reviewed here, the interaction of our field with the theory of stellar structure and evolution continued, but mostly in another form. The intensive search for collapsed objects (black holes and neutron stars) — as the final stages of stellar evolution for more massive stars — naturally focused attention on binary systems, for a black hole can reveal its presence only by its gravitational field.