In the wake of the Great Recession, a new cycle of scholarship opened on the history of American capitalism. This occurred, however, without much specification of the subject at hand. In this essay, I offer a conceptualization of capitalism, by focusing on its root—capital. Much historical writing has treated capital as a physical factor of production. Against such a “materialist” capital concept, I define capital as a pecuniary process of forward-looking valuation, associated with investment. Engaging recent work across literatures, I try to show how this conceptualization of capital and capitalism helps illuminate many core dynamics of modern economic life.