Marx’s theory of surplus-value is fundamental to his innovations in the theory of value and Classical Political Economy. When Marx’s theory of surplus-value is considered in the context of the long-period method, the dynamics of surplus-value and its importance to Marx’s overall framework can be properly presented. This approach reveals that Marx’s use of an equalized rate of surplus-value across sectors of production in Volume III of Capital is not merely a convenient assumption. The equalization of the sectoral rate of surplus-value is in fact one of the central tendencies of Marx’s framework, and is elevated to the level of an economic law by Marx. The reasoning behind Marx’s use of an equalized rate of surplus-value is the mobility of labor found in Adam Smith. This reasoning, when combined with the long-period method, reveals that the rate of surplus-value across sectors is subject to the same turbulent dynamics and equalization process as the rate of profit, and should not be deviated from when applying Marx’s vision.