This synthesis summarizes information available in the scientific literature on historical patterns and contemporary changes in fuels and fire regimes in mountain big sagebrush communities. This literature suggests that presettlement fires in the sagebrush biome were both lightning- and human-caused. Peak fire season occurred between April and October and varied geographically. Wildfires were high-severity, stand-replacement fires. Fire frequency estimates range from decades to centuries, depending on the applicable scale, methods used, and metrics calculated. Fire frequency was influenced by site characteristics. Because mountain big sagebrush communities occur over a productivity gradient driven by soil moisture and temperature regimes, fire regimes likely varied across the gradient, with more frequent fire on more productive sites that supported more continuous fine fuels. Sites dominated by mountain big sagebrush burned more frequently than sites dominated by Wyoming big sagebrush, because the former tend to be more productive. Mountain big sagebrush communities adjacent to fire-prone forest types (e.g., ponderosa pine) may have had more frequent fires than those adjacent to less fire-prone types (e.g., pinyon-juniper) and those far from forests and woodlands. Most fires were likely small (less than ~1,200 acres (~500 ha)), and large fires (>24,000 acres (10,000 ha)) were infrequent. Large fires were most likely after one or more cool, wet years that allowed fine fuels to accumulate and become more continuous. Since European-American settlement, fuel and fire regime characteristics in many big sagebrush communities have shifted outside the range of historical variation. Settlement generally began in the mid-1800s and caused changes in ignition patterns and fuel characteristics, although the timing and magnitude of these changes varied among locations. Since then, fuels and fire regimes in many sagebrush ecosystems have changed due to a combination of interrelated factors, including land development for agriculture and energy, urbanization and infrastructure development, proliferation of nonnative invasive plants, woodland expansion, overgrazing by livestock, fire exclusion, and climate changes. Since 1980, the number of fires each year and total annual area burned have increased in the sagebrush biome. However, in most mountain big sagebrush communities, available data suggest that fire frequency has either not changed or has been reduced, with the exception of an area in the Colorado Plateaus ecoregion where fire frequency may have increased due to frequent prescribed burning.