  We review the fundamentals and the recent developments in understanding of
common envelope physics. We report specifically on the progress that was made
by the consideration of the recombination energy. This energy is found to be
responsible for the complete envelope ejection in the case of a prompt binary
formation, for the delayed dynamical ejections in the case of a self-regulated
spiral-in, and for the steady recombination outflows during the transition
between the plunge-in and the self-regulated spiral-in. Due to different ways
how the recombination affects the common envelope during fast and slow
spiral-ins, the apparent efficiency of the orbital energy use can be different
between the two types of spiral-ins by a factor of ten. We also discuss the
observational signatures of the common envelope events, their link a new class
of astronomical transients, Luminous Red Novae, and to a plausible class of
very luminous irregular variables.
