We cannot know the future perfectly, but we can imagine the future and make plans for it by making some assumptions—or judgments— about what that future could be like. Some of the assumptions we make are pretty likely to come true; others are more vulnerable to uncontrollable and unforeseen events; still others seem quite unlikely. Some of the assumptions are likely to be very important to the success of the plan; others will be more peripheral. Assumption-Based Planning (ABP) is a tool designed for improving the robustness and adaptability of plans—for reducing the number of avoidable surprises in any plan or planning. It is primarily a “post-planning” tool (recognizing that planning is an iterative process) that concentrates on the assumptions in an already-developed plan that are most important to the plan's success and that are most uncertain. Specifically, ABP works to decrease the risks that assumptions represent.