A sensitivity analysis is a powerful technique for understanding systems. This heuristic paper shows how to overcome some of the difficulties of performing sensitivity analyses. It draws examples from a broad range of fields: bioengineering, process control, decision making and system design. In particular, it examines sensitivity analyses of tradeoff studies. This paper generalizes the important points that can be extracted from the literature covering diverse fields and long time spans. Sensitivity analyses are particularly helpful for modeling systems with uncertainty.