Glycosylation of proteins shows two striking features: first, there are a large number of glycan structures that are involved in various biological events (diversity) and, second, certain structures are selectively added to specific sites within proteins (specificity) in a context-dependent manner. However, the mechanisms that regulate diversity and specificity are unknown. Over the last few decades, genetic studies in Drosophila have been used to examine the functions mediated by glycosylation in vivo. Recent improvements in the combined use of biochemical/biophysical, genetic, and cell biology approaches has allowed the rapid and detailed analysis of the structure, function and formation of glycans in Drosophila. This chapter aims to provide an overview of the different types of glycans and their biological function(s) in Drosophila. The regulation of glycan synthesis in Drosophila and mammals is also discussed. © 2012 by Nova Science Publishers, Inc. All rights reserved.