Explanations are essential in the teaching process. Tracers are one possibility to provide students with explanations in an intelligent tutoring system. Their development can be divided into four steps: (a) the definition of the trace model; (b) the extraction of the information from this model; (c) the analysis and abstraction of the extracted information; and (d) the visualization of the result of the analysis. This article describes the architecture of a domain independent tracer developed in Visual Prolog displaying explanations in the form of a textual dialog or a graphical tree. (Contains 12 figures.)