Glaciers once covered 30% of Earth’s land area, leaving diverse landforms; glacial geologists study glaciers to understand the formation of these landforms. Glacier ice is a metamorphic rock deforming at temperatures close to the melting point; structural geologists study glaciers to learn more about the origin of similar structures in other rocks. Ice cores from glaciers contain a well-dated record of climatic fluctuations over millennia, so climatologists study glaciers to understand the drivers of Earth’s climate. Failure of glacier dams can cause floods that engineers and town officials seek to prevent. Anthropogenically induced global warming is causing retreat of ice sheets and mountain glaciers; planners and policy makers want to know how to stop this retreat, and how fast it will raise sea level, impacting coastal infrastructure. A quantitative understanding of the physics of glaciers is essential for rigorous analysis of many of these problems. Glaciers occur in spectacular remote areas, unscarred by human activities; these environments appeal to many glaciologists.