A topic map, like a database or XML document, provides a way of structuring information and as such requires methods for specifying the rules that describe the allowed states of the map. This article discusses the need for a mechanism for defining constraints on topic maps to make sure that topic maps are semantically valid according to the intents of the topic map designer. As the paper will show the need for a constraint system is very much there. Examples of constraints are given in order to give the readers some ideas of how far such a constraint system could be taken. Existing constraint mechanisms are mentioned and their applicability for describing constraints on topic maps are discussed.