Among the wide variety of amylolytic enzymes synthesized by microorganisms, alpha-amylases are the most widely used biocatalysts in starch saccharification, baking industries and textile desizing. These enzymes randomly cleave the alpha-1,4-glycosidic linkages in starch, generating maltose and malto-oligosaccharides. The commercially available alpha-amylases have certain limitations, such as limited activity at low pH and Ca2+-dependence, and therefore, the search for novel acid-stable and thermostable amylases from extremophilic microorganisms and the engineering of the already available enzymes have been the major areas of research in this field over the years. Several attempts have been made to find suitable microbial sources of acid-stable and thermostable alpha-amylases. Acid-stable alpha-amylases have been reported in fungi, bacteria and archaea. alpha-Amylases that are active at elevated temperatures have been reported in bacteria as well as in archaea. alpha-Amylases that possess both characteristics, to the extent required for their various applications are very scarce. The developments that have been made in molecular biology, directed evolution and structural conformation studies of alpha-amylases for improving their properties to suit various industrial applications are discussed in this review. (C) 2013 Elsevier Ltd. All rights reserved.