Robert M. Augé's review discusses the impact of vesicular-arbuscular mycorrhizal (VAM) fungi on water relations in host plants, both well-watered and drought-stressed. VAM symbiosis can alter water movement into, through, and out of host plants, affecting tissue hydration and leaf physiology. The review summarizes published findings and suggests underlying mechanisms. Previous reviews by various authors are cited. Early studies suggested VAM effects on water relations were mainly nutritional, but later research showed that VAM can influence water relations independently of phosphorus (P) nutrition. Studies in the 1980s confirmed these findings, showing that VAM plants can exhibit different water relations compared to non-mycorrhizal (NM) plants, even when size and P nutrition are similar. Two main camps emerged: those supporting strictly nutritional effects and those acknowledging non-nutritional effects. Recent studies suggest that VAM fungi can modify host water relations in ways unrelated to improved P acquisition. VAM symbiosis, host phenology, P and carbon nutrition are closely linked, so most field effects of VAM fungi on host water balance are likely partially related to modified plant size or developmental rates. Over 200 peer-reviewed articles have been published on VAM fungi's influence on water relations, photosynthetic rates, and drought responses of 90 host species. Five genera of mycorrhizal fungi, including 22 species of Glomus, have been studied, mostly with herbaceous hosts in controlled environments. VAM and NM plants often display different transpiration rates and stomatal conductances. VAM effects on stomatal conductance have been observed under both well-watered and drought conditions. VAM symbiosis can affect stomatal sensitivity to atmospheric water status. The review does not cover salinity stress.Robert M. Augé's review discusses the impact of vesicular-arbuscular mycorrhizal (VAM) fungi on water relations in host plants, both well-watered and drought-stressed. VAM symbiosis can alter water movement into, through, and out of host plants, affecting tissue hydration and leaf physiology. The review summarizes published findings and suggests underlying mechanisms. Previous reviews by various authors are cited. Early studies suggested VAM effects on water relations were mainly nutritional, but later research showed that VAM can influence water relations independently of phosphorus (P) nutrition. Studies in the 1980s confirmed these findings, showing that VAM plants can exhibit different water relations compared to non-mycorrhizal (NM) plants, even when size and P nutrition are similar. Two main camps emerged: those supporting strictly nutritional effects and those acknowledging non-nutritional effects. Recent studies suggest that VAM fungi can modify host water relations in ways unrelated to improved P acquisition. VAM symbiosis, host phenology, P and carbon nutrition are closely linked, so most field effects of VAM fungi on host water balance are likely partially related to modified plant size or developmental rates. Over 200 peer-reviewed articles have been published on VAM fungi's influence on water relations, photosynthetic rates, and drought responses of 90 host species. Five genera of mycorrhizal fungi, including 22 species of Glomus, have been studied, mostly with herbaceous hosts in controlled environments. VAM and NM plants often display different transpiration rates and stomatal conductances. VAM effects on stomatal conductance have been observed under both well-watered and drought conditions. VAM symbiosis can affect stomatal sensitivity to atmospheric water status. The review does not cover salinity stress.