ZDDP (zinc dialkyldithophosphate) is one of the most successful lubricant additives, introduced over 60 years ago and still widely used in engine oils. Despite efforts to replace it, no cost-effective alternative with comparable antiwear performance has been found. ZDDP has also inspired extensive research due to its multifunctional role as an antioxidant, corrosion inhibitor, and antiwear agent. This paper reviews the history of ZDDP as a lubricant additive, from its development in the late 1930s to the present. It assesses current understanding of ZDDP's mechanisms, noting that while much is known about its antiwear film properties, the reaction pathways and kinetics of film formation and removal remain unclear. The review focuses on research studying ZDDP in simple solutions, as this clarifies its mechanisms better than studies in complex formulated oils. The paper also highlights important research questions about ZDDP's behavior as an antiwear additive. ZDDP's development followed other lubricant additives, with oiliness additives first introduced in 1918 and other classes developed between 1930 and 1940. Interest in phosphorus-containing additives, especially those with sulfur, grew in the late 1930s, as shown by US patent data. The paper concludes that ZDDP remains a crucial component of motor oils and that further research is needed to fully understand its mechanisms.ZDDP (zinc dialkyldithophosphate) is one of the most successful lubricant additives, introduced over 60 years ago and still widely used in engine oils. Despite efforts to replace it, no cost-effective alternative with comparable antiwear performance has been found. ZDDP has also inspired extensive research due to its multifunctional role as an antioxidant, corrosion inhibitor, and antiwear agent. This paper reviews the history of ZDDP as a lubricant additive, from its development in the late 1930s to the present. It assesses current understanding of ZDDP's mechanisms, noting that while much is known about its antiwear film properties, the reaction pathways and kinetics of film formation and removal remain unclear. The review focuses on research studying ZDDP in simple solutions, as this clarifies its mechanisms better than studies in complex formulated oils. The paper also highlights important research questions about ZDDP's behavior as an antiwear additive. ZDDP's development followed other lubricant additives, with oiliness additives first introduced in 1918 and other classes developed between 1930 and 1940. Interest in phosphorus-containing additives, especially those with sulfur, grew in the late 1930s, as shown by US patent data. The paper concludes that ZDDP remains a crucial component of motor oils and that further research is needed to fully understand its mechanisms.