Carbohydrate structures, also known as glycans, are ubiquitous in biology. Cells from all living organisms are coated with glycosylated molecules that dictate extracellular recognition events, and glycosylated molecules play important intracellular roles as well. Despite many critical functions in processes as diverse as development, immunology, cell signaling, neurobiology, and infectious disease, methods to study glycan function lag behind approaches applied to other biomolecules such as proteins and nucleic acids. To facilitate identification of glycan binding partners, we have prepared photocrosslinking sugar analogs of two monosaccharides – sialic acid and N-acetylglucosamine (GlcNAc) – and devised strategies to incorporate these unnatural sugars into cellular glycoconjugates in place of their normal counterparts. Subsequent ultraviolet irradiation of intact cells results in covalent crosslinking of the glycan and its binding partner(s). The covalent complexes can be analyzed by a variety of approaches including immunoblot and mass spectrometry-based proteomics to define glycan-based recognition events that occur in a cellular setting. I will describe the methods that we developed for photocrosslinking sugar incorporation as well as two applications of this technology – (1) defining host cell receptors for cholera toxin and (2) understanding the mechanistic basis of signaling by the intracellular O-GlcNAc modification.