Aluminum (Al) toxicity is one of the major constraints for agriculture on acid soils, which occupy large regions of the word's agricultural area. At the low pH values associated with these soils (pH 4 -5). Al+3 is solubilized into the soil solution and is toxic to plants, dramatically inhibiting root growth and crop yield. Plant breeding approaches have been used to develop Al tolerant crop genotypes for over 50 years. In order to further improve crop Al tolerance, researchers are now investigating the molecular and physiological mechanisms underlying crop Al tolerance, with the ultimate goal of isolating major Al tolerance genes. Sorghum is an important cereal crop species that exhibits extraordinary levels of tolerance to abiotic stresses such as Al toxicity and drought. Yet, almost nothing is known about the genetics and physiology of sorghum Al tolerance, hence this was the focus of the thesis research described here. A detailed molecular genetic characterization of the mechanisms of Al tolerance in sorghum was conducted in two sources of sorghum Al tolerance, SC283 and SC566. SC283 was found to harbor a single major Al tolerance gene (Alt sb), which exhibited partial dominance in this study. Alt sb was subsequently mapped to a chromosomal location that was apparently not conserved in relation to the location of known Al tolerance genes in wheat and barley. Additional studies indicated that Al tolerance in SC566 is also controlled by the Alt sb gene. The results are discussed with regards to the possible conservation of Al tolerance genes across species in the Poaceae. The physiological studies of Al tolerance were performed with Al tolerant (ATF10B) and sensitive (ATF8B) near isogenic lines derived from SC283. These studies suggested that Alt sb encodes a tolerance mechanism that acts by excluding Al from sensitive sites in the root tip via and Al-activated root citrate exudation. Al tolerance in ATF10B appears to be inducible by Al, in that Al tolerance increased as the roots were exposed to Al for longer time periods. The thesis is concluded with a description of future research that will focus on the isolation of Alt sb by positional cloning.