Our interest in ice sheets and glaciers stems from their acting as freshwater storages and potential peril of alpine and coastal habitations as well as from variations of their areal extent and volume in response to climate changes. Also, they serve as an archive of the Earth’s climate history. The Antarctic and Greenland ice sheets play a major role in the global climate by regulating the sea level and modifying the atmospheric general circulation. The influence of glaciers is limited to the local climatic and hydrological conditions. In this chapter, the basic definitions are first given, and then the structure and classification of glaciers are described, along with a discussion of their geographical distribution and mass balance. Then, the fundamental physical laws governing glacial ice flow, the conservation of mass and momentum, are derived from the basic principles and presented with their applications. Finally, essential aspects of hydrology and water circulation inside and at the perimeters of ice sheets and glaciers are presented, including the calving of icebergs, followed by the introduction to iceberg evolution and drift.