GEOMORPHOLOGY IS the study of landforms. Landforms are surface expressions of rocks, as are various features made by rivers, groundwater, waves and currents, winds, glaciers, and corals. Landforms come in all shapes and sizes. A delta, plateau, VOLCANO, sinkhole, and beach are all landforms. Geomorphologists are geologists and physical geographers who specialize in the study of landforms. Geomorphology has two general goals: 1) to explain how landforms vary from place to place; and 2) to develop theories about the origin and development of landforms. In order to achieve these goals, geomorphologists examine the nature of surface rocks and geologic processes, such as soil formation, weathering and erosion, mass movements, and transportation and deposition of sediments. They use a wide range of techniques for data collection, including field, laboratory, and numerical techniques. Geomorphologic research aids in understanding the role that landform development plays in complex ecosystems. The findings also help prepare for and lessen impacts of hazardous geological events, such as landslides, floods, beach erosion, and slope erosion. The following paragraphs summarize how geologic structure, plate tectonics, gradational processes, and time influence landform development. The four factors are interrelated so that their combined effects create an amazing variety of beautiful landforms for us to enjoy. The article concludes with a summary of how geomorphic theory in the United States evolved from a long tradition of geological and geographical research.