In human and animal nutrition, much interest has been focused on the potential role of dietary supplements in promoting health, athletic performance and disease mitigation. Supplements may include essential nutrients provided in amounts greater than required to prevent a deficiency state, or substances purported to have a role in metabolism or tissue function but that are not recognized as an essential nutrient. This review aims to provide the rationale and scientific evidence for use (or not) of some of the supplements marketed for use in horses, with emphasis on supplements purported to directly boost performance, such as creatine, carnitine and branched-chain amino acids. It also discusses the so-called ‘joint supplements’ (or slow-acting, disease-modifying osteoarthritis agents), such as glucosamine and chondroitin sulphate. The effects of selected feeding strategies on performance, including fat supplementation, are also examined. It is concluded that although the use of nutritional supplements is commonly alleged to boost performance or health in horses, for most, if not all, of these supplements there is little or no scientific evidence of efficacy.