The proximal isovelocity surface area (PISA) measurement, also known as the "flow convergence" method, can be used in echocardiography to estimate the area of an orifice through which blood flows. It has many applications, but this review focuses only on its use in the intraoperative evaluation of mitral regurgitation. In that setting, PISA provides a quantitative assessment of the severity of mitral regurgitation and it is useful in clinical decision-making in the operating room. In this review, I discuss the physical principles behind the PISA method, along with the various mathematical formulas used to calculate the effective mitral regurgitant orifice area, the regurgitant volume, and the regurgitant fraction. A step-by-step approach is presented and illustrated with graphic and video demonstrations. Finally, I will discuss the various limitations and technical considerations of PISA measurement in the operating room.