Proteoglycans, with their core proteins and attached glycosaminoglycan chains, are recognized as important partners in many biological processes, yet often experimental analysis of their molecular action is considered for only part of these molecules: either the protein or the carbohydrate unit. In this article, we have tried to summarize, with an example of the syndecan family in general and more specifically with syndecan-1, what is known considering the mutual influence of these different components, and we follow whether the nature of the glycosaminoglycan chains matters for these effects.