An expanding giant star may engulf a nearby stellar or substellar companion. The common envelope phase that follows, fundamentally alters the evolution of both stars in the system by reducing the orbital separation and leading to a merger (such as V838 Mon or V1309 Sco), or a compact binary (e.g., novae, type Ia supernova progenitors, X-ray binaries). Frequencies and physical properties of all compact, evolved binaries depend sensitively on the poorly understood common envelope phase. We have developed 3- dimensional, hydrodynamic common envelope simulations between a red giant branch star and a range of companions. Comparing the modelled ejected masses and final separations with observations, we revisit our understanding of the common envelope efficiency, a parameter on which predictions such as supernova type Ia rates sensitively rest. Finally, we bring our results to bear on the surprising result that some planets can survive a common envelope phase with their mother star.