AbstractChondroitin sulfate (CS) is one of the most evolutionarily conserved glycosaminoglycans (GAGs). Although CS's function in skeletal development is well established in vertebrates, CS exists in more primitive animal species with no cartilage or bone, such as C. elegans and Drosophila, indicating that the original role of CS was not in the skeletal system. In this review, we focus on the roles of CS and the mechanisms of action during development of two genetically trackable model organisms, C. elegans and Drosophila.