Publisher Summary This chapter provides an overview of aspartate ammonia-lyase, which catalyzes the reversible conversion of L-aspartic acid to fumarate and ammonia. Enzyme synthesis in Escherichia coli W cells is subject to catabolite repression by glucose and is suppressed under aerobic conditions. The assay method routinely employed for this enzyme is based on the spectrophotometric determination of fumarate formed. The steps of the purification procedure are preparation of crude extract, streptomycin treatment, heat treatment, ammonium sulfate fractionation, and calcium phosphate gel treatment. Several properties related to the aspartate ammonia-lyase enzyme are reviewed in the chapter. Some of them are substrate specificity, K m values, activators and inhibitors, and stability. Because the substrate saturation profiles exhibit complex kinetics including positive or negative cooperativity as a function of pH and other factors, exact K m values are hardly estimated. The enzyme is fairly stable in the presence of high concentrations of inorganic salts, such as ammonium sulfate, potassium phosphate, and KCl.