In this innovative book, Gloria Frost reconstructs and analyses Aquinas's theories on efficient causation and causal powers, focusing specifically on natural causal powers and efficient causation in nature. Frost presents each element of Aquinas's theories one by one, comparing them with other theories, as well as examining the philosophical and interpretive ambiguities in Aquinas's thought and proposing fresh solutions to conceptual difficulties. Her discussion includes explanations of Aquinas's technical scholastic terminology in jargon-free prose, as well as background on medieval scientific views - including ordinary language explanations of the medieval physical theories which Aquinas assumed in formulating his views on causation and causal powers. The resulting volume is a rich exploration of a central philosophical topic in medieval philosophy and beyond, and will be valuable especially for scholars and advanced students working on Aquinas and on medieval natural philosophy.