Abstract: Many computer conferencing systems based on the X Window System have recently emerged. While these systems hold the promise for fostering collaboration among groups of geographically separated individuals, they are, at present, difficult to build because the X programming model does not support conferencing. This paper describes the salient problems that face the designers of X-based shared window systems and provides solutions and implementation principles for addressing the problems. The enumeration of issues and solutions is based on our experiences with XTV -- an X-based shared window system we have developed.