Since the Sixth American College of Chest Physicians Consensus Conference on Antithrombotic Therapy, the results of clinical trials have provided important new information on the management of thromboembolic disorders, and the science of developing recommendations has advanced. In the accompanying supplement, we provide the new and previously existing recommendations and review several important changes that we have made in our guideline development process. We made a conscious effort to increase the participation of female authors and of contributors from outside North America, with the latter reflecting the widespread use and dissemination of these guidelines internationally. The change in the title from a conference emphasizing consensus to “ACCP Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines” reflects the evidence-based approach to making recommendations. The recommendations follow the grading system described in the 2001 recommendations. If the guideline developers are very certain that benefits do, or do not, outweigh risks, burdens, and costs, they will make a strong recommendation (in our formulation, Grade 1). If they are less certain of the magnitude of the benefits and the risks, burdens, and costs, and thus of their relative impact, they make a weaker Grade 2 recommendation. Consistent results from RCTs generate Grade A recommendations, observational studies with very strong effects or secure generalizations from randomized clinical trials (RCTs) generate Grade C+ recommendations, inconsistent results from RCTs generate Grade B recommendations, and observational studies generate Grade C recommendations. We now use the language “we recommend” for strong recommendations (ie, Grades 1A, 1C+, 1B, and 1C) and “we suggest” for weaker recommendations (ie, Grades 2A, 2C+, 2B, and 2C). While evidence on which recommendation are made remains weak in the fields of pediatric thrombosis, thrombosis in pregnancy, and thrombosis in valvular heart disease, rigorous studies in other fields have resulted in new and strong evidence-based recommendations for many indications.