Formation of endogenous thrombi in blood vessels is one of the leading causes of death in our modern life. According to data provided by the World Health Organization (WHO) in 2000, heart diseases are responsible for 29% of the total mortality rate in the world. For this, a tremendous amount of research has been done in the area of prevention and treatment of these diseases. The classical therapy of these thrombi relies upon the use of antiplatelets, anticoagulants, or even surgeries. Relatively recently, the fibrinolytic enzymes produced by microorganisms, snakes, earthworms, insects, plants, and other organisms are being successfully used in the treatment of blood clots, especially with regard to the direct dissolving action on fibrin in tandem with less cost and side effects in comparison with the first-generation thrombolytic agents, streptokinase and urokinase. Furthermore, recombinant DNA technology has succeeded in improving and decreasing the undesirable effects of the first generation of enzymes. Recombinant PAs or rt-PAs like alteplase, retelase, saruplase, tenecteplase, lanoteplase, and desmoteplase became available in the drug markets with advantages of less binding loci with PAI-1 to avoid degradation while providing faster and more complete reperfusion in a greater number of patients with less risk of bleeding and intracranial hemorrhage. This review is the first to cover all the natural and recombinant thrombolytic agents used in enzyme therapy. (c) 2014 American Institute of Chemical Engineers Biotechnol. Prog., 30:656-672, 2014