Mitral valve disease is common. Mitral regurgitation is the second most frequent indication for valve surgery in Europe and despite the decline of rheumatic fever in western societies, mitral stenosis of any aetiology is a regular finding in all echo departments. Mitral valve disease is therefore one of the most common pathologies encountered by echocardiographers, as both a primary indication for echocardiography and a secondary finding when investigating other cardiovascular disease processes. Transthoracic and transoesophageal echocardiography (TOE) play a crucial role in the assessment of mitral valve disease and are essential to identifying the aetiology, mechanism and severity of disease and for helping determine the appropriate timing and method of intervention. This guideline, from the British Society of Echocardiography (BSE), describes the assessment of mitral regurgitation and mitral stenosis and replaces previous BSE guidelines describing the echocardiographic assessment of mitral anatomy prior to mitral valve repair surgery and percutaneous mitral valvuloplasty. It provides a comprehensive description of the imaging techniques (and their limitations) employed in the assessment of mitral valve disease. It describes a step-wise approach to identifying: aetiology and mechanism, disease severity, reparability and secondary effects on chamber geometry, function and pressures. Advanced echocardiographic techniques are described for both transthoracic and transoesophageal modalities, including TOE and exercise testing.