In software development a lack of certainty about something can be part of the solution rather than part of the problem. This point of view can, however, seem a little counterintuitive and more than a little disturbing. There is a strong tendency for humans to feel unsure about uncertainty, in two minds over ambiguity and a little wobbly with instability. Whether over technology choice, implementation options, requirements or schedule, uncertainty is normally seen as something you must either suppress or avoid. Of this many people appear, well, certain. That you should embrace it and use it to help determine schedule and design is not immediately obvious. This session looks at how uncertainty can be employed in planning and used constructively to help determine object encapsulation, class hierarchy design and package partitioning.