Interventional cardiologists have long navigated the balance between ischemic and bleeding risks in patients with coronary artery disease, from the dawn of thrombolysis therapy to today’s drug-eluting stent era. Historically, bleeding events were not systemically reported in clinical trials, leading to varied reported incidences. With the recognition of poor outcomes after bleeding events, the definitions of bleeding also transformed alongside the evolution of antithrombotic treatments and strategies like abbreviated dual-antiplatelet therapy. The GUSTO and the original TIMI classifications mark the inception of dedicated bleeding criteria from the thrombolysis trials with many inherent limitations; these classifications were succeeded by multiple classifications in the stent era with increasing granularity. However, the lack of a common language made the cross-comparison and interpretation complex and confusing. To address this, the bleeding academic research consortium (BARC) definition finally emerged, which can identify more bleeding events without compromising the prognostic value in multiple validation studies. In this chapter, we delved into the development of different bleeding classifications, compared their validations in prospective cohorts, and explored the future directions for bleeding definition.