Glycosaminoglycans are complex polysaccharides exhibiting a large structural and conformational diversity. These key biological players organize the extracellular matrix, contribute to cell-matrix interactions, and regulate cell signaling. Natural and synthetic libraries of glycosaminoglycans have been spotted on microarrays to find glycosaminoglycan partners, and determine the size and the chemical groups promoting protein binding. Advances in glycosaminoglycan sequencing allow the characterization of glycosaminoglycan sequences interacting with proteins, and glycosaminoglycan-mediated pull-down proteomics can identify glycosaminoglycan binding-proteins at a proteome scale in various biological samples. The analysis of the glycosaminoglycan interaction networks generated using these data gives insights into the molecular and cellular mechanisms underlying glycosaminoglycan functions. These interactomes can also be used to design inhibitors targeting specific GAG interactions for therapeutic purpose.