Because the X window system is so general and flexible, developers require some form of higher-level toolkit in which to implement applications. Such a toolkit has to address a number of difficult issues, at several different levels. In this panel, designers of the three most widely used X tool kits, the X Toolkit, the Andrew Toolkit, and InterViews, describe in retrospect the features of their toolkit architectures which were successful and those which, for various reasons, were not. They then discuss what lessons future toolkit designers should learn from their experience. The intent is not to pit the different toolkits against each other, but to view each of them as an experiment which can yield insights for future use.