Duration is traditionally used to measure the exposure to interest rate risk of a fixed income portfolio and the contribution of the individual components to its overall risk. Although the duration of a financial instrument undoubtedly represents an indispensable parameter for the quantification and the management of financial risk, the attempt to extend the concept of duration to equities has proven very complex and has often been a source of confusion. However, given the importance of this parameter in financial immunization, risk management, tactical asset allocation, and since it could be a relevant factor even in estimating the cost of equity capital, equity duration certainly cannot be set aside. In this chapter, the main equity duration models and the most relevant empirical results reported in academic literature are illustrated in detail. In addition, since equity duration has been the subject of considerable debate among academics and scholars over the years, the so-called equity duration paradox is discussed.