A growing body of evidence suggests that low molecular weight heparin (LMWH) is at least as effective as unfractionated heparin (UFH) in the medical management of acute coronary syndromes (ACS) including acute myocardial infarction. Several studies support an early invasive as compared to a conservative (non-invasive) approach utilizing percutaneous coronary intervention (PCI) in these patients. During coronary angiography and PCI systemic anticoagulation usually administering UFH should prevent thrombus formation at the catheter equipment and at the site of vessel injury. The widespread use of UFH as the anticoagulant of choice is explained by low costs, easy reversibility and accepted anticoagulant properties. However, the clinical support for the use of UFH in PCI is largely based on retrospective and observational data only. Compared to UFH, LMWH have distinct pharmacological advantages, including ease of administration and usually no need for monitoring. A major drawback of LMWH is the predominant renal clearance which may lead to unpredictable levels of anticoagulation in patients with impaired renal function. Since LMWH constitute an extremely heterogeneous group of drugs each LMWH should be recognized as an individual pharmaceutical compound. Consequently, pharmacodynamic and pharmacokinetic properties of one LMWH must not be extrapolated for other LMWH. A growing body of evidence supports the use of LMWH in ACS, with or without GPIIb / IIIa receptor antagonists, thrombolysis, or PCI. Although in patients undergoing PCI, LMWH have yet not been shown to be superior to UFH they can safely be administered.