This chapter has been prepared to assess the contributions of host resistance to improved productivity of alfalfa forage. The hypothesis addressed states that host resistance is an important determinant of forage yields by alfalfa cultivars as demonstrated by a significant positive relationship between increasing levels of resistance and higher yields of forage. Regression of pest resistance ratings versus forage yields over a 3-yr duration using data from 1 experiment in each of 15 states indicates a significant (P = 0.10) positive relationship when ratings for insect and plant-pathogen resistance are combined. A summary of results from several tests conducted in Montana and Oklahoma indicates that annual yield increases with multiple-pest-resistant cultivars over susceptible standards may range from 0.3 to 1.8 Mg/ha. Additional studies conducted in Oklahoma have shown impressive increases in stand retention and forage yield with multiple-pest-resistant cultivars over susceptible standards through 6 yr of production. Although it is likely that some proportion of yield improvements may be caused by enhanced climatic adaptation or other factors leading to greater yield potential, our analyses have provided convincing evidence that host resistance is an important contributor to yield increases recorded with multiple-pest-resistant alfalfa cultivars.