The Allium genus includes approximately 500 species. Commonly used allium vegetables include garlic, onion, leeks, chives, scallions which are used all over the world in different delicacies. Some allium vegetables have been employed for millenia in the traditional medical practice to treat cardiovascular diseases. They have been shown to have applications as antimicrobial, antithrombotic, antitumor, hypolipidaemic, antiarthritic and hypoglycemic agents. In recent years, extensive research has focused on the anticarcinogenic potential of allium vegetables and their constituents, viz., allylsulfides and flavonoids (particularly quercetin which is present abundantly in onion). Epidemiological studies have shown that higher intake of allium products is associated with reduced risk of several types of cancers. These epidemiological findings are well correlated with laboratory investigations. Organosulfur compounds present in Allium vegetables, are considered to be responsible for the beneficial effects of these herbs. Several mechanisms have been proposed to explain the cancer-preventive effects of Allium vegetables and related organosulfur compounds. These include inhibition of mutagenesis, modulation of enzyme activities, inhibition of DNA adduct formation, free-radical scavenging, and effects on cell proliferation and tumor growth. Although there is a large body of evidence supporting these mechanisms, they are still speculative, and further research is needed to support causality between such properties and cancer-preventive activity in experimental animals. This article reviews current knowledge concerning allium vegetables and cancer prevention.