Low molecular weight heparin (LMWH) and unfractionated heparin (UFH) are used to prevent rethrombosis and distal platelet embolisation in acute coronary syndromes. LMWH have a more predictable anticoagulant response and are less likely to result in bleeding. For the moment UFH should be used in primary percutaneous coronary intervention (PCI). It may also be preferable to use UFH in the setting of rescue PCI following tenecteplase (TNK) treatment. In those over 75 years of age, the combination of TNK with enoxaparin has been shown to be superior to TNK with UFH in reducing ischaemic end points without increasing the risk of haemorrhage. Results from TIMI IIB indicate that enoxaparin is superior to UFH for the acute management of non-ST elevation ACS (in patients managed conservatively). Enoxaparin and UFH appear to have similar efficacy and safety profiles when used in conjunction with glycoprotein IIb/IIIa blockade during PCI.