This chapter discusses the mitral regurgitation is the most prevalent valvular heart disease. The most appropriate management remains difficult and controversial. Patients with primary MR are frequently asymptomatic despite important remodeling and reduction in contractile function or reserve. Patients with secondary MR have a poor outcome, and treatment does not necessarily improve long-term prognosis. Echocardiography, through its numerous modalities, plays an important role in the diagnosis, assessment and follow-up of this condition. Etiology, mechanisms of regurgitation, quantification of MR severity, MR repercussions, feasibility of mitral valve repair, and dynamic component are described in detail. 14 comprehensive and self-explanatory images are included in this chapter. And 49 references are given at the end of the chapter to facilitate further reading.