The paper presents taxonomy of criteria and procedures for evaluating software development tools used in safety-critical real-time systems. The ultimate purpose of the research is to provide a base for creation of guidelines for the tool certification process. The specific application area is airborne software and appropriate references are made to the accepted RTCA DO-178B guidelines. The software design and coding processes are focal point of tool assessment. The paper presents the industry viewpoint, tool qualification and evaluation criteria, and an active experiment serving as a test-bed for collecting software development effort data and engineering observations supporting the tool assessment methodology.