Prescribed burning as a fuel treatment seeks to moderate wildfire impacts and decreases the areal extent of wildfires by increasing the effectiveness of fire suppression. Assessment of prescribed burning effectiveness is frequently anecdotal or based on simulation. This paper examines recent observational evidence of prescribed fire effectiveness. The spread rate and intensity of experimental fires in distinct fuel types have been shown to increase with time since treatment (fuel age) following fuel structure recovery. Prescribed fire constrains the size and especially the severity of individual fires, even under extremely severe weather conditions. At larger spatial and temporal scales of analysis, the effect of fuel age on unplanned fire severity is also evident, whether it comes from wildfires entering treated areas or from wildfires in fuel-reduced areas resulting from earlier wildfire occurrences. The persistence of these effects is variable, depending on vegetation type and productivity. The long-term reduction in wildfire area brought about by prescribed fire can be difficult to ascertain. Substantial effort (annual treatment rates >5% of the landscape) has been shown to effectively control the extent of wildfires in forests, with 3-4 units of prescribed burning needed to reduce wildfire by one unit. Future studies should consider the decrease in area burned by high-severity fire as a more meaningful and objective measure of prescribed fire effectiveness than the decrease in wildfire area and should strive to document fire behaviour in treated versus untreated areas during wildfires.