This chapter will cover the functions, properties, and composition of automatic transmission fluids (ATFs), along with a brief description and history of original equipment manufacturer (OEM) ATF specifications for passenger cars. The cited works include an eclectic mixture of materials designed to give the reader interested in further study some historic and some more modern information, along with information from different points of view when possible. By no means is this meant to be a comprehensive review; it is only meant to be a small window into the world of transmission lubricant technology.