The common envelope phase in the evolution of binary systems is examined. Three parameters are identified which characterize the efficiency of energy deposition, the importance of three-dimensional effects, and the efficiency of spin-up of the envelope. It is demonstrated that the efficiency of deposition of orbital energy into envelope ejection can be quite low. It is found that significant spin-up of the envelope can be expected to occur in relatively early stages, when the spiralling-in occurs inside evolved supergiant envelopes. In normal giants spin-up can occur only in the final stages of the spiralling-in process. The results of a simplified three-dimensional numerical calculation of the common envelope phase are presented, and the implications of the results for the formation of planetary nebulae with binary nuclei, double white dwarf systems, and FK Com stars are discussed. 52 references.