Functional mitral regurgitation (MR) is a common clinical entity which will likely increase in the future due to predicted demographic changes. It is also associated with poor long-term survival. The anatomic structure of the mitral valve apparatus is complex and consists of several components, each of which can be affected by a variety of diseases resulting in MR. In primary MR, the valvular incompetence is caused by compromised or structurally disrupted components of the valve apparatus; the valve in functional MR is structurally normal, with the regurgitation resulting from failure of coaptation of the mitral valve leaflets without coexisting structural changes of the valve itself. Therefore, we defined functional MR as a systolic retrograde flow from the left ventricle into the left atrium due to reduction and/or elimination of the normal systolic coaptation of the mitral valve leaflets. A slow progression of the symptoms is typical for this valve disease and often ends in irreversible left ventricular dysfunction. The pathophysiology and treatment of functional MR are quite complex. This article reviews and summarizes the existing literature, with a focus on the pathophysiology and current treatment of functional MR.