Allium vegetables, such as garlic, have been used for medicinal purposes throughout the recorded history. The known health benefits of Allium vegetables constituents include cardiovascular effects, improvement of the immune function, lowering of blood glucose level, radioprotection, protection against microbial infections, and anti-cancer effects. Initial evidence for the anti-cancer effect of Allium vegetables was provided by population-based case-control studies. Subsequent laboratory studies showed that the Allium vegetable constituents, such as diallyl disulfide, S-allylcysteine, and ajoene can not only offer protection against chemically induced cancer in animal models by altering carcinogen metabolism, but also suppress growth of cancer cells in culture and in vivo by causing cell cycle arrest and apoptosis induction. Suppression of angiogenesis and experimental metastasis by Allium constituents has also been reported. Defining the mechanism by which sulfur compounds derived from Allium vegetables inhibit cancer cell growth has been the topic of intense research in the last two decades. Some Allium vegetable constituents have also entered clinical trials to assess their safety and anti-cancer efficacy. This article summarizes preclinical and limited clinical data to warrant further clinical evaluation of Allium vegetable constituents for prevention and therapy of human cancers.