Energy loss is a well-established engineering concept that when applied to evaluating the performance of native heart valves and valvular prostheses has the potential for providing valuable information about the impact of valve function on myocardial performance. The concept has been understood for many years, but its routine application has been hindered not only by a lack of understanding of its meaning but also because of the lack of investigational tools to easily obtain the data necessary for its estimation. Today the gathering of that information is becoming easier, and thus the time has come to revisit the efficacy of energy loss for evaluating heart valve performance. This review defines what energy loss is, how it is measured, and how it might be applied to clinical situations of heart valve disease to better understand the impact of valvular disease on ventricular function.