Erosion is the weathering away of the surface of Earth (or other planetary body) by natural agents. Running water constitutes the most effective eroding agent, the process being accelerated by the transportation of particles eroded or weathered farther upstream: it is these that are primarily responsible for further erosion. Groundwater may cause erosion by dissolving certain minerals in the rock. Ocean waves and especially the debris that they carry may substantially erode coastlines. Glaciers are extremely important eroding agents, eroded material becoming embedded in the ice and acting as further abrasives. Many common landscape features, such as drumlins and fjords, are the results of glacial erosion.

Rocks exposed to the atmosphere undergo weathering: mechanical weathering usually results from temperature changes (for example, in exfoliation, the cracking off of thin sheets of rock due to extreme daily temperature variation); chemical weathering results from chemical changes brought about by, for example, substances dissolved in rain water. Wind erosion may be important in dry, sandy areas.