Glycogenosis type II (GSDII, Pompe disease) is an autosomal recessive lysosomal storage disease caused by a deficiency of acid α-glucosidase (acid maltase, GAA). The enzyme degrades a -1,4 and α -1,6 linkages in glycogen, maltose, and isomaltose. Deficiency of the enzyme results in accumulation of glycogen within lysosomes and in cytoplasm eventually leading to tissue destruction. The discovery of the acid a-glucosidase gene has led to rapid progress in understanding the molecular basis of glycogenosis type II and the biological properties of the GAA protein. The last decade has seen several developments: 1) extensive mutational analysis in patients with different forms of the disease, 2) characterization of the enzyme biosynthesis, processing, and lysosomal targeting, 3) generation of knockout mouse models, 4) development of viral vectors for gene replacement therapy, 5) the production of recombinant human enzyme, and 6) a shift in the enzyme replacement therapy approach from theory to practice. It is anticipated that the enzyme replacement therapy will be widely available for human use in the near future. Several recent reviews (including the most comprehensive one by R. Hirschhorn and A. Reuser ), address clinical, biochemical and genetic aspects of the disease, as well as development of new therapies for GSDII [2, 3, 4]. In this article we will review recent findings in the area including rapidly accumulating molecular genetic data (more than 20 mutations need to be added to the list), transcriptional control of gene expression, studies in mouse models, and new approaches to gene therapy. We will also highlight some emerging questions following the introduction of enzyme replacement therapy.