Polyols, also known as sugar alcohols, are chemically reduced forms of aldose or ketose sugars. Many vascular plants make and metabolize polyols such as mannitol and sorbitol as direct products of photosynthesis together with sucrose. In addition to serving as carbon and energy sources, these versatile carbohydrates are potent antioxidants that also protect plants against salt and osmotic stress. In recent years, research has largely focused on the transport of polyols, particularly mannitol, in plants. At the same time, a number of major breakthroughs have been made in elucidating the roles of mannitol in fungi; in particular the role of fungal mannitol in plant-pathogen interactions, and the corresponding defensive role of the pathogen induced mannitol catabolic enzyme mannitol dehydrogenase in plants. In this article we will provide a brief overview of polyol metabolism in higher plants and then focus on these areas of recent progress.