The International Society on Thrombosis and Haemostasis (ISTH) recently published a survey on the management of unfractionated heparin (UFH), which revealed substantial heterogeneity in practice and highlighted the need to develop evidence-based guidance. This document summarizes evidence and provides consensus guidance, developed through a Delphi process, for preanalytical, analytical, and clinical aspects of UFH management. For preanalytical issues, optimal practice includes centrifuging blood samples within 1 hour of collection and testing within 10 minutes of centrifugation, although a 1-hour interval is acceptable. Standard 3.2% (109 mM) buffered trisodium citrate tubes are recommended, with no routine use of CTAD (citrate, theophylline, adenosine, and dipyridamole) tubes due to cost and uncertain benefit. Analytically, anti-Xa assays are preferred over activated partial thromboplastin time for UFH monitoring, as they are less affected by interferences and do not require the determination of a local range. Assays without dextran sulphate or added antithrombin are optimal, especially to avoid overestimating UFH activity after protamine reversal. Establishing a local heparin therapeutic range is essential if activated partial thromboplastin time is used. Clinically, a target anti-Xa level of 0.30 to 0.70 U/mL is suggested for most indications, including venous thromboembolism, arterial events, and mechanical heart valves. Weight-based nomograms are recommended for dosing, although no single nomogram is preferred due to limited outcome data. Antithrombin supplementation is generally not advised for acquired deficiency. The guidance highlights critical evidence gaps and calls for further research to optimize UFH monitoring and dosing, with future documents addressing special populations and periprocedural management.