Fisher volcano, containing the largest Holocene caldera in the Aleutian volcanic arc, is an active volcano near the center of Unimak Island, about 120 kilometers southwest of Cold Bay and about 175 kilometers northeast of Dutch Harbor. The volcano is composed of numerous small volcanic centers around and within a large, oval caldera 12 by 18 kilometers in diameter and 500 to 1,000 meters deep that formed during a catastrophic eruption about 9,400 years ago. Since then, more than 30 separate vents inside and outside the caldera have erupted; the most recent eruption occurred in 1826. These eruptions have produced lava flows and widespread tephra (volcanic ash) deposits, and have occasionally been accompanied by large floods and lahars. The history of Fisher volcano includes repeated explosive and non-explosive eruptions of varying levels of intensity, and indicates that this pattern will continue in the future. Understanding the variety of potential hazards associated with different types of volcanic processes typical of Fisher volcano can help minimize the adverse effect of future eruptions on humans and infrastructure. This report is part of a series of volcano hazard assessments being prepared by the Alaska Volcano Observatory. The reports are intended to describe the nature of volcanic hazards at Alaska volcanoes and show the extent of hazardous areas with maps, photographs, and other appropriate illustrations. The reports are preliminary and subject to revision as new data become available.