There has been a significant effort to understand the events that occur when two or more droplets collide. The majority of the early work, which dates from the 1960s, focused on droplet growth relating to precipitation and was thus limited to water droplet collision phenomena. Recently, studies have sought information on the collision behavior of fuel droplets for application to combustion spray systems. Researchers have found distinct differences between the collision behavior of water droplets and fuel droplets. The extensive experimental data on the droplet collision process is reviewed and presented here for both water and fuel droplet collisions. Collision outcomes of bounce, temporary coalescence followed by separation or catastrophic fragmentation and permanent coalescence are examined and an effort is made to relate the existing findings to a unified description of collisional droplet behavior.