A fundamental problem in control theory is the stability of a given linear system. The design of a control system is generally based on a simplified model. The true values of the physical parameters may differ from the assumed values. This monograph addresses stability problems for linear systems with parametric uncertainty. The application of convexity techniques leads to computationally tractable stability criteria for families of characteristic functions with nonlinear dependence on the parameters. Stability results as well as stability criteria for time-delay systems with uncertainties in coefficients and delays are reported.