We propose to use Constraint Logic Programming (CLP) for the specification and implementation Qualitative Reasoning (QR) problems that are spedalized Constraint Satisfaction Problems. The use of CLP has two advantages: (i) CLP gives a well defined and understood logical framework for the problem specification, and (ii) CLP is not only a logical framework, it is also a family of languages specially developed for solving classes of CSP problems. Thus we obtain a class of powerful implementation languages for rap!d prototyping. To illustrate the steps of specification and implementation we describe in detail the core of the QSIM algorithm (Knipers 1994), namely the filtering of the state transitions in the CLP framework. We show how the basic constraints are specified in this framework and describe the technical aspects of an implementation. We want to demonstrate the advantages of CLP through an example for a large and complex qualitative reasoning algorithm.