In this article we review the biological effects of Al, primarily with respect to the chemical factors controlling Al bioavailability and toxicity, and how its biological effects are best predicted. Our intent is not to duplicate recent reviews on Al chemistry or toxicity, but rather to update the literature since these reviews were published, and to focus on Al speciation and other external chemical influences on Al bioavailability to freshwater biota. Briefly, we first review Al chemistry, with a specific focus on understanding, as well as measuring, Al chemical species of importance to aquatic biota. Next we more comprehensively review Al toxicity and bioavailability to freshwater algae, with a thorough analysis of the relationships between speciation and toxicity, the role of important chemical complexing agents such as P, Si, and organic carbon, as well as the potential for Al to impact algal community structure. A third section reviews the more sparse literature on aquatic higher plants; the fourth section reviews a somewhat more abundant literature of Al toxicity to freshwater invertebrates. We close with an updated review of Al toxicity to fish, again with a focus on mechanisms of toxicity, and the role of Al speciation in controlling bioavailability.