Left atrial (LA) spontaneous echo contrast (SEC), or "smoke," is a frequent finding on transesophageal echocardiography (TEE), but it is rarely detected with transthoracic echocardiography. LA SEC is characterized by dynamic smoke-like echoes within the LA cavity or appendage. Most patients with LA SEC have atrial arrhythmias, mitral stenosis, or a mitral valve prosthesis, and they have an enlarged LA, conditions that are associated with LA stasis. Conversely, mitral regurgitation is protective against LA SEC. LA SEC is present in almost all patients with LA thrombus and is associated with previous embolic events in many patient populations. In patients with nonvalvular atrial fibrillation, LA SEC predicts future embolism and death. LA SEC may therefore assist in selecting patients with atrial fibrillation or with mitral stenosis and sinus rhythm who benefit the most from anticoagulation. Hematological studies have shown that LA SEC is a marker of an hypercoagulable state. LA SEC is a manifestation of red cell aggregation, arising from an interaction between red cells and plasma proteins such as fibrinogen, at low shear rates. LA SEC does not require platelets. The detection of LA SEC on ultrasound arises from the increased amplitude of backscatter from red cell aggregates rather than single cells. Patients with LA SEC should be considered for anticoagulant therapy and may require correction of underlying cardiovascular abnormalities. Future directions in LA SEC include further assessment of integrated backscatter for quantification, assessment of its prognostic role in clinically low-risk patients with nonvalvular AF, and novel pharmacological treatment.