Systems optimization is an exciting new tool for improving the performance of existing comminution plants while incurring minimal capital expenditures and producing high rates of return. The goal of this approach is to achieve significant performance gains without installing new equipment or replacing older, less efficient equipment. Thus, rates of return can be much higher than the historical rates of return in the process industries. This paper describes a methodology for systems optimization that has produced significant rates of return for several comminution plants. The approach seeks to determine important cost-sensitive, synergistic interactions in a plant, find the optimum conditions that improve the performance of the whole plant and accurately identify the tradeoffs between incremental costs and additional revenue. The interactions are described by mathematical models of the processes occurring in the system which can then be programmed into plant simulators. Two case studies of comminution plant optimization which illustrate the concepts of systems optimization are presented in this paper. The first case study concerns a system optimization project for the SAG mill circuit at a western US gold mine. The financial rate of return for that project was 145%, The second case study is a systems optimization project with a rate of return of 115% for a large iron ore concentrator in Europe.