A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. / A new approach to the problem ofdesigning a control system for a run-of-mine (ROM) milling circuit is described. The ROM mill control problem is formalized in terms or a general linear system synthesis and analysis framework. Using this framework, u-synthesis and analysis is applied to a model of an industrial ROM milling circuit. The milling circuit uncertainty structure is obtained from plant perturbation tests. Correlations of plant parameter uncertainties and the choice of the location of uncertainty weights within the plant transfer function matrix structure, are discussed. Some comments are made as to possible sources of the plant uncertainties, and practical measures that can be taken to minimize them. The choice of performance weighting functions is discussed with regard to the economic and process-related importance of each plant output and issues of implementation such as the sampling rate to be used. A u-controller is obtained which provides robust performance in the face of significant model uncertainties. This controller is tested on a real industrial ROM milling circuit It is shown that the u-controller could be made to work on an industrial plant. Practical problems encountered are discussed, and reasons are given as to why the behaviour of the physical system differs form that observed in computer simulations. (Abbreviation abstract) / AC 2018