Stellar mergers and common envelope phases represent dramatic moments of stellar interaction in stellar lifetimes. Despite their presumed existence for nearly 50 years, the details of these events have remained uncertain. We are now learning that these events can lead to bright transients, which have been observed both in galactic and extragalactic contexts. I will present some features and questions posed by this emerging class of transients. I will discuss how these observations inform hydrodynamic modeling of binary interactions, and what can be learned at the interface between models and observations of common envelope phases "in action."