  The common envelope phase of binary star evolution plays a central role in
many evolutionary pathways leading to the formation of compact objects in short
period systems. Using three dimensional hydrodynamical computations, we review
the major features of this evolutionary phase, focusing on the conditions that
lead to the successful ejection of the envelope and, hence, survival of the
system as a post common envelope binary. Future hydrodynamical calculations at
high spatial resolution are required to delineate the regime in parameter space
for which systems survive as compact binary systems from those for which the
two components of the system merge into a single rapidly rotating star. Recent
algorithmic developments will facilitate the attainment of this goal.
