Plants of the genus Aloe have perhaps the longest recorded history of medicinal usage and are amongst the most widely used plants for traditional medicinal purposes worldwide. Aloe vera, Aloe ferox, Aloe arborescens and Aloe perryi are the best known and most widely used, but many other species are also used for their therapeutic properties. The Aloes have been used since ancient times, particularly for the treatment of microbial infections, gastrointestinal disorders and inflammatory conditions. In addition to their myriad uses in traditional therapeutics, the Aloes have also been used as components of cosmetic formulations, and in the food and beverage industries. Despite their wide acceptance, studies from different laboratories often report wide variations in the therapeutic bioactivities from within the same Aloe species, even when the same extraction procedures are used. Furthermore, leaves from individual Aloe plants within the same species may have widely varying levels of the bioactive phytochemicals. Phytochemical analyses have shown that many Aloe species contain various carbohydrate polymers (notably glucomannans) and a range of other low molecular weight phenolic compounds including alkaloids, anthraquinones, anthrones, benzene and furan derivatives, chromones, coumarins, flavonoids, phytosterols, pyrans and pyrones. There has been a wealth of information published about the phytochemistry and therapeutic potential of the Aloes (especially Aloe vera). Much of this has been contradictory. Intra- and interspecies differences in the redox state of the individual Aloe components and in the ratios of these components may occur between individual plants. These factors may all affect the physiological properties of Aloe extracts. Due to the structure and chemical nature of many of the Aloe phytochemicals, it is likely that many of the reported medicinal properties are due to antioxidant or prooxidant effects. The antioxidant/prooxidant activities of many Aloe phytochemicals depend not only on their individual levels, but also on the ratios between the various components and their individual redox states. Therefore, discrepancies between bioactivity studies are likely when using different crude mixtures. This report aims to summarise the phytochemistry of the Aloes and (a) examine how their constituents may be responsible for their medicinal properties and (b) some possible reasons for the wide variations reported for their medicinal properties and (c) their therapeutic mechanisms. Some future areas of research into the medicinal activities of this important genus are also highlighted.