Aquinas thinks that not all instances of efficient causation are equivalent. Certain instances of efficient causation, namely per se cases, are the most fundamental and proper. Other instances of efficient causation happen in virtue of these cases. The chapter reconstructs Aquinas’s views on per se efficient causation. The chapter next examines Aquinas’s views on the temporal and modal relationship that obtains between per se natural efficient causes and their effects. The chapter shows that Aquinas thought that per se causes are simultaneous with their effects. Contemporary scholars debate whether Aquinas thought that natural efficient causes necessitate their effects. The chapter brings greater clarity to Aquinas’s views by examining his distinctions between different types of natural efficient causes and different types of necessity. Finally, the chapter considers Aquinas’s views on important relational conditions for efficient causation: the agent and the patient must be distinct and they must be in contact with each other. The chapter analyzes Aquinas’s arguments against self-motion and action at a distance.