This project took place from June 1996 to October 1996 at CWI, Amsterdam in the Interactive System Department. It is a former part for the award of the MSc. in Information Engineering at Sheffield Hallam University This report describes the work done on the study and the design of a visual debugging tools for the Visifold programming language. A survey on existing tools and the different techniques involve in the process of debugging. This leads us toward a fast prototyping of DebFold, showing the feasibility of our technical choices. ii Acknowledgments This project would not have been possible without the agreement of Paul ten Hagen, head of the IS Department. Thank you to Farhad Arbab project leader for the Manifold team, who let me work with his group. Many thanks to Pascal Bouvry, my project supervisor and my friend for many years, who has always been available to all my questions and arguing. I thank Freek Burger, Kees Everaars and Kees Blom for helping me whenever I needed it..