The aim of this review paper is to show the most important bioactive compounds of garlic (Allium sativum L.), modes of action of garlic powder and its effects on broiler chicken productive performance. Allicin is thought to be the principal bioactive compound present in aqueous garlic extract or raw garlic homogenate. When garlic is chopped or crushed, the allinase enzyme present in garlic is activated and acts on alliin to produce allicin. Other important sulphur containing compounds present in garlic are allyl methyl thiosulphonate, 1-propenyl allyl thiosulphonate and γ-L-glutamyl-S-alkyl-L-cysteine. These active compounds are thought to influence feed consumption, feed utilisation, body weight, blood lipid profiles and cholesterol levels in chickens. Biological responses to these bioactive compounds have been attributed to a reduction in risk factors for cardiovascular diseases and cancer, stimulation of immune function, enhanced detoxification of foreign compound, hepatoprotection, antimicrobial, antifungal, anti-inflammatory and antioxidant effects.