Atrial function is an integral aspect of the proper performance of the circulatory system. Anatomical and hemodynamic assessment of left and right atrial function can be performed by two-dimensional echocardiography and tissue Doppler imaging. This chapter discusses the echocardiographic tools for the assessment of atrial function and the effect of percutaneous mitral valvuloplasty on atrial function. Patients with mitral stenosis who still preserve their sinus rhythm often show left atrial dilatation and systolic dysfunction in the body of the left atrium and left atrial appendage (LAA), a circumstance which may constitute the anatomic-functional substrate for thrombus formation, with the ensuing risk of systemic thromboembolism requiring preventive therapeutic steps.