A thrombus is an important component of the active and unstable atherosclerotic coronary plaque. A thrombus exhibits unique physical properties and commonly displays profound abnormal vascular behavior. The presence of a thrombus has been recognized as a strong predictor of significant risk for complications and poor outcome in percutaneous interventions and imposes technical challenges regarding its removal from the vasculature. Thus, angiographic detection of a thrombus serves as a procedure hallmark, which requires careful consideration for selection of an appropriate treatment approach. The grading of an underlying thrombus content and its burden on coronary flow can be assessed by various classifications. The aim of this article is to describe the pathophysiologic components of thrombus formation and to delineate the role of corresponding contemporary classifications that can be used for its assessment. Incorporating thrombus classification into the decision-making process in percutaneous coronary revascularization can enhance the yield of procedures and ensure high-quality interventions.