Massive stars are the cosmic engines of the universe, driving the chemical enrichment and mechanical evolution of galaxies. A large fraction of massive stars are found in binary systems, and interactions between these stars can fundamentally alter the evolutionary paths of both stars. One of the most critical phases in the evolution of massive binary stars is the contact phase, where both stars fill their Roche lobes and share a common envelope. The contact phase represents a crossroad in the evolution of massive binary stars.  Depending on the internal physics, the predicted end products can vary greatly including various exotic objects such as Be stars, magnetic massive stars, LBVs, peculiar Type-II supernovae, and gravitational wave sources. Nearly a quarter of all massive stars will evolve through a contact phase at some point during their lifetimes, however, despite its importance, large uncertainties exist in our understanding of the internal physics and the final evolutionary outcome of this phase. This is due to both the complex interaction physics and a lack of observational constraints: only 13 massive contact binaries are currently known.

Despite the small sample size, massive contact binaries can provide vital observational constraints to the various evolutionary pathways that involve binary mergers. In this talk, I will discuss the current state of the field of massive overcontact binaries, with a specific focus on the internal mixing processes during this phase.  I will discuss the theoretical predictions as well as what the observational data tells us, and how these compare and contrast with one another.  I will also describe a new spectroscopic analysis technique specifically designed to analyze these highly deformed systems and I will discuss how accounting for the 3D geometry can change our understanding of these objects. Finally I will discuss the future direction of the field and how we can attempt to bridge the gap between theory and observations in the coming years.