Acquired diseases of the mitral valve include mitral insufficiency and mitral stenosis. The further classification of mitral insufficiency into primary and secondary forms is of great importance for treatment. In primary MI, a valve component is structurally nonfunctional and causes the insufficiency, most often a prolapse of the leaflets. In secondary MI, the valve itself is intact, but an underlying dilation of the left ventricle (usually due to cardiomyopathy) or the left atrium leads to a reduced coaptation of the valve leaflets and subsequently to insufficiency. In severe, symptomatic primary MI, valve reconstruction is the treatment of choice, while in secondary MI, the treatment of the underlying (systolic) heart failure is the leading therapy strategy. The classic mitral stenosis is rare due to the now low incidence of rheumatic fever. In industrialized countries, the degenerative-calcifying form is increasing. In rheumatic MS, percutaneous mitral valve commissurotomy is the first choice for initial treatment if suitable anatomical criteria are present. Otherwise, a surgical mitral valve replacement is indicated. Important in terms of drug therapy is sufficient anticoagulation (with vitamin K antagonists!), as the flow limitation in the left atrium in mitral stenosis creates a thrombogenic milieu.