Info-gap decision theory is a method for analysis, planning, modeling, decision or design under uncertainty. The future may differ from the past, so our models may err in ways we cannot know. Our data may lack evidence about surprises: catastrophes or windfalls. Our scientific and technical understanding may be incomplete. These are info-gaps: incomplete understanding of the system being managed as it bears on decisions to be made. Info-gap theory provides decision-support tools for modelling and managing severe uncertainty. Three ideas are central in an info-gap analysis: robustness to uncertainty, satisfying critical requirements, and exploiting favorable opportunities. I will briefly describe info-gap theory and discuss a simple example.