The Three Sisters are three adjoining volcanic peaks in central Oregon whose summits all rise above 10,000 feet, making them among the state's highest mountains and a dominant feature of the Cascade Range. Though long treated as a single landmark, North, Middle, and South Sister are separate volcanoes with different ages and eruptive histories, shaped by volcanism, glaciers, and repeated cold periods. North Sister is the oldest and most eroded, Middle Sister is the smallest and least studied, and South Sister is the youngest and only one known to have erupted in the last 14,000 years. Their glaciers, snowfields, hazards, and ongoing monitoring make them important both scenically and scientifically.