Tribology is a term which arose in the 1960s to describe the study of interacting moving surfaces involving friction and wear. Tribology encompasses aspects of physics, chemistry, applied mathematics, material science, mechanical and chemical engineering and applied mechanics. By applying the science of tribology to practical problems, such as reduction of friction and wear, innovative strides are being made towards molecular level knowledge and understanding in the transportation industry. This book focuses on a group of molecules called zinc dialkyl-dithiophosphates (ZDDPs) which have been used as engine oil additives for over 50 years. They are important chemically-active additives, known for their antioxidant and antiwear characteristics. ZDDPs are known to form a protective film on surfaces commonly used in the automotive industry. Neither the mechanism of this film formation nor the origins of its efficacy are well understood, this is due to the complex nature of the engine environment. This work focuses on what chemical species are formed once the molecule breaks down, and we determine and correlate the mechanical properties.