This book inquires into the origin and early development of Roy Harrod's notion of economic dynamics. It examines how Harrod gathered the analytical, methodological and epistemic components of his theory, and how these are logically connected. It shows that the organizing concept is the instability principle, a premise rather than a result of Harrod's trade cycle theory. The relationship of Harrod's dynamics with the 'orthodox theory' and with the alternative approaches to dynamics is also examined.