A binary star system consists of two stars that orbit a common center of mass, rather than orbiting each other in a simple planetary fashion. The gravitational interaction between the components is key to understanding the stellar evolution of both objects. The physical relationship between the stars determines how mass is distributed and exchanged; if the stars are close enough, gravitational forces can lead to mass transfer, wherein material from one star accretes onto the other, which can dramatically alter the observed properties of the system over astronomical timescales. These systems provide critical observational grounds for studying stellar dynamics, angular momentum transfer, and the processes by which heavy elements are dispersed throughout the cosmos.