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Onion (Allium cepa L.) is botanically included in the Liliaceae and species are found across a wide range of latitudes and altitudes in Europe, Asia, N. America and Africa. World onion production has increased by at least 25% over the past 10 years with current production being around 44 million tonnes making it the second most important horticultural crop after tomatoes. Because of their storage characteristics and durability for shipping, onions have always been traded more widely than most vegetables. Onions are versatile and are often used as an ingredient in many dishes and are accepted by almost all traditions and cultures. Onion consumption is increasing significantly, particularly in the USA and this is partly because of heavy promotion that links flavour and health. Onions are rich in two chemical groups that have perceived benefits to human health. These are the flavonoids and the alk(en)yl cysteine sulphoxides (ACSOs). Two flavonoid subgroups are found in onion, the anthocyanins, which impart a red/purple colour to some varieties and flavanols such as quercetin and its derivatives responsible for the yellow and brown skins of many other varieties. The ACSOs are the flavour precursors, which, when cleaved by the enzyme alliinase, generate the characteristic odour and taste of onion. The downstream products are a complex mixture of compounds which include thiosulphinates, thiosulphonates, mono-, di- and tri-sulphides. Compounds from onion have been reported to have a range of health benefits which include anticarcinogenic properties, antiplatelet activity, antithrombotic activity, antiasthmatic and antibiotic effects. Here we review the agronomy of the onion crop, the biochemistry of the health compounds and report on recent clinical data obtained using extracts from this species. Where appropriate we have compared the data with that obtained from garlic (Allium sativum L.) for which more information is widely available.