Spacecraft autonomy is becoming an increasingly important technology. Yet the very nature of autonomy - on-board decision making and largely unattended operation - makes it important that such systems be thoroughly tested and validated. We are developing software visualization tools to assist in the validation process. The tools, which are designed to facilitate human problem solving, combine graphical layout and color to present “gestalt” views of system execution, together with interactive facilities for browsing, searching, and tracking down potential problems. This paper describes two tools being developed in conjunction with the NASA New Millennium Program - one for visualizing inter-process comm-unication and one for visualizing plan execution.