Nonuniform contrast opacification of vasculature is frequently encountered on thoracic computed tomographic angiography. The purpose of this pictorial essay is to discuss the appearance of, and factors underlying mixing artifacts, which we term "smoke." We provide an approach to distinguish it from pathology including pulmonary embolism, aortic dissection, and thrombus. Smoke results from a combination of technical factors, abnormal physiology, or inflow of unopacified blood. Smoke produces ill-defined filling defects that may be confidently diagnosed in many cases if these fundamentals are applied.