Herbicide-resistant annual ryegrass (Lolium rigidum) is having a dramatic impact on the management and profitability of continuous cropping systems of southern Australia. In many cases ryegrass populations have exhibited cross resistance. where development of resistance through repeated use of one herbicide also bestows resistance to other groups of herbicides not previously used. Hence use of the major groups of selective herbicides as the predominant means of weed control is often ruled out. In this paper we discuss the development of herbicide resistance in Lolium rigidum, its costs and implications for farm management. Where high levels of resistance have developed, weed control strategies that include a range of control methods (such as increased crop densities, windrowing and crop topping) give the optimal return.