When a process model differs from the true process, the closed-loop qualities of a model-based control algorithm such as Generic Model Control are in doubt. The conditions under which stability of the closed-loop GMC system is guaranteed (“robust stability”) and the performance of the closed loop system is guaranteed to meet predetermined performance objectives (“robust performance”) are given for the first time in terms of the model and its uncertainty description. The GMC parameters which give the best performance can be determined through a simple optimisation procedure. The analytical techniques are illustrated through a simple example.