A long standing ecological paradigm suggests that diverse plant communities should be more resistant to weed invasion compared with plant communities that have few resident species. Some data from native and naturalized grasslands support this idea, but less is known about whether such relationships exist in more intensively managed pasturelands. Weeds are a major management issue pasturelands and finding ways to reduce their abundance without excessive herbicide application has obvious significance. My talk will summarize some research we have conducted on relationships between resident plant diversity in pasturelands and weed invasion. Resident plant diversity is a function of two components - species richness and evenness. Through regional surveys and small plot work, we have found little relationship between resident species richness and weed invasion in pasturelands. A more critical component of pasture diversity appears to be the evenness of resident species. In several instances, we have found significant negative relationships between resident species evenness and weed invasion in pastures. Still though, resident species diversity explains only some of the variation in weed invasion success. In manipulative experiments, we have found resident plant species composition (e.g., legume vs. grass) can also have a significant effect on invasion resistance. Species composition, in turn, strongly interacts with disturbance and resource availability to influence weed invasion. Overall, our findings suggest weed invasion in pasture communities is not a simple function of resident plant diversity. Weed invasion, instead, is likely driven by several interacting factors within the resident plant community that include: disturbance, resource availability, species composition and the size of the weed species at specific times during the invasion process.