Developing controllers for hydraulic servos is difficult due to the inherent uncertainty and nonlinear characteristics of the system. Systems are classed as uncertain when they are subject to unknown parameter variations or disturbances, or when there is incomplete knowledge of the system model, all of which are common in hydraulic servo systems. However, unlike conventional control techniques, the use of modern control methods means that system uncertainty can be considered at the controller design stage, and consequently robust controllers can be developed. In this paper the robust control of hydraulic servo systems is considered, and sources of uncertainty typical in hydraulic drives are discussed. The effect of ignoring these uncertainties is demonstrated by simulation experiments of a hydraulic servo, comparing conventional control techniques to the modern approach of H∞ optimal control. The implementation of modern controllers is also discussed.