At least half of the stars are in binaires. Binary interactions make stellar evolution more complicated, and lead to the formation of many important stellar objects. In this talk, I will discuss mass transfer in binary systems and common envelope evolution, which determines the evolutionary paths and destinies of the systems. I will also discuss the formation of progenitors of type Ia supernovae, hot subdwarf binaries, low-mass white dwarf binaries, double compact objects as gravitational wave sources. Finally I will discuss the application of binary evolution to the study of galaxies.