Uncertainty is an inherent feature of all real-life complex systems. It can be described in different forms; we focus on the parametric description. The simplest results on stability of linear systems under parametric uncertainty are the Kharitonov theorem, edge theorem, and graphical tests. More advanced results include sufficient conditions for robust stability with matrix uncertainty, LMI tools, and randomized methods. Similar approaches are used for robust control synthesis, where performance issues are crucial.