Functional mitral regurgitation (FMR) develops frequently in ischemic or dilated cardiomyopathy. It is clinically very important because it is strongly associated with the progression of the disease and its prognosis. Although the correct mechanism of the FMR is not fully understood, incomplete mitral leaflet closure area or tenting area is the most important determinant of the FMR. Regional or global remodeling, and the contractile dysfunction of the left ventricle could affect tenting area. However, there is still a debate about which is the most important factor among them to determine the tenting area. Also, new findings in histopathology of the mitral valve leaflets with the FMR suggest leaflet itself could be involved in the development of the FMR. For the treatment of the FMR, conventional mitral annuloplasty with or without ring is not always successful in some patients, and it may not prevent further remodeling of the left ventricle even in successfully repaired cases. Therefore, it needs more sophisticated and prospective study to evaluate the accurate mechanism of the FMR, and new treatment modality should be developed for the successful treatment of the functional mitral regurgitation.