A multi-species version of the bio-economic RIM (Resistance and Integrated Management) model has been developed to deal with the complexities involved in the long-term integrated management of annual ryegrass (Lolium rigidum Gaud.) and wild radish (Raphanus raphanistrum L.), which dominate and co-exist in southern Australia. In this paper, we present a review of the existing options on how to model multi-species competition in order to select the best approach for incorporation in the RIM framework. Furthermore, we show how we have extended the original single-species ryegrass RIM model to include other aspects of the wild radish biology as well as a set of extra weed management practices used to control this weed species. We also demonstrate how the Multi-species RIM model can be used to evaluate weed management scenarios of co-existing herbicide resistant species. This is done through investigating the implications of using Roundup Ready® canola in the system.