A mudflow, also known as mudslide or mud flow, is a form of mass wasting involving fast-moving flow of debris and dirt that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second. Mudflows contain a significant proportion of clay, which makes them more fluid than debris flows, allowing them to travel farther and across lower slope angles. Both types of flow are generally mixtures of particles with a wide range of sizes, which typically become sorted by size upon deposition.

Mudflows are often called mudslips, a term applied indiscriminately by the mass media to a variety of mass wasting events. Mudflows often start as slides, becoming flows as water is entrained along the flow path; such events are often called mud failures.

Other types of mudflows include lahars (involving fine-grained pyroclastic deposits on the flanks of volcanoes) and jökulhlaups (outbursts from under glaciers or icecaps).

A statutory definition of "flood-related mudslide" appears in the United States' National Flood Insurance Act of 1968, as amended, codified at 42 USC Sections 4001 and following.

Heavy rainfall, snowmelt, or high levels of groundwater flowing through cracked bedrock may trigger a movement of soil or sediments in landslides that continue as mudflows. Floods and debris flows may also occur when strong rains on hill or mountain slopes cause extensive erosion and/or mobilize loose sediment that is located in steep mountain channels. The 2006 Sidoarjo mud flow may have been caused by rogue drilling.

The point where a muddy material begins to flow depends on its grain size, the water content, and the slope of the topography. Fine grained material like mud or sand can be mobilized by shallower flows than a coarse sediment or a debris flow. Higher water content (higher precipitation/overland flow) also increases the potential to initiate a mudflow.

After a mudflow forms, coarser sediment may be picked up by the flow. Coarser sediment picked up by the flow often forms the front of a mudflow surge and is pushed by finer sediment and water that pools up behind the coarse-grained moving mudflow-front. Mudflows may contain multiple surges of material as the flow scours channels and destabilizes adjacent hillslopes (potentially nucleating new mudflows). Mudflows have mobilized boulders 1–10 m across in mountain settings.

Some broad mudflows are rather viscous and therefore slow; others begin very quickly and continue like an avalanche. They are composed of at least 50% silt and clay-sized materials and up to 30% water. Because mudflows mobilize a significant amount of sediment, mudflows have higher flow heights than a clear water flood for the same water discharge. Also, sediment within the mudflow increases granular friction within the flow structure of the flow relative to clear water floods, which raises the flow depth for the same water discharge. Difficulty predicting the amount and type of sediment that will be included in a mudflow makes it much more challenging to forecast and engineer structures to protect against mudflow hazards compared to clear water flood hazards.