Among the most pronounced vegetation changes in past 130 years has been the increase in both distribution and density of juniper (Juniperus spp.) and pinyon (Pinus spp.) across the Intermountain West. Juniper and pinyon species between the Canadian and Mexican borders occupy over 30 million ha throughout this region. Prior to European settlement, woodland species were primarily confined to rocky ridges or surfaces where sparse vegetation limited fire. Woodlands now occupy more productive sites with deeper well-drained soils. Woodland species began their unprecedented and ongoing rates of increase during the late 1800s. Replacement of sagebrush shrub steppe, riparian, and aspen (Populus spp.) communities by pinyon and juniper species is largely attributed to the reduced occurrence of fire. An important sagebrush type that has been impacted by recent woodland expansion in mountain big sagebrush (Artemisia tridentata var. vaseyana). Prior to settlement, mean fire return intervals for a large portion of this cover type were 12-25 years. At present, fire return intervals in this cover type have increased to > 100 years. As trees gain dominance and shrubs and herbaceous vegetation decline, fuel structure changes, which contributes to significant increases in the length of mean fire return intervals. Fire safe communities successionally replace fire-dependent communites. However, in the central and southern portions of the Intermountain West, particularly where pinyon is dominant, dense tree-canopied woodlands are now becoming susceptible to intense crown fires. The intensity of these fires can lead to dominance by exotics, further altering the successional dynamics of the site. During the past, juniper and pinyon woodlands have been treated to control the expansion. However, wildlife and environmental concerns, and different perceptions of the intrinsic values of these environments have recently limited treatment of woodlands, including the use of prescribed fire. During the early to middle stages of development when woodlands contain understories of native shrubs and herbs, they can successfully be treated by various methods, particularly fire. However, once communities become tree-dominated woodlands, treatment becomes difficult and expensive.