This article describes the design and formative evaluation of a Web-based tool that supports curriculum developers in constructing competence maps. Competence maps describe final attainment levels of educational programs in terms of--interrelated--competencies. Key requirements for the competence-mapping tool were validity and practicality. Validity refers to internal consistency and meaningful links to the external realities represented. Practicality refers to a design approach of evolutionary prototyping, in which feedback from intended users and domain experts is collected throughout the development process. Formative evaluations of four prototypes were conducted. Measures of design, appeal, goal, content, confidence and relevance showed that the tool is practical. The article describes the formative evaluation process and concludes with a description of the modified tool from the perspective of the user and the instructional designer.