This paper presents a formalization for Topic Maps™.We first simplify TMRM, the current ISO standard proposal for a TM reference model and then characterize topic map instances. After defining a minimal merging operator for maps we propose a formal foundation for a TM query language. This path expression language allows us to navigate through given topic maps and to extract information. We also show how such a language can be the basis for a more industrial version of a query language and how it may serve as foundation for a constraint language to define TM-based ontologies.