Gravity-driven flows of material emanating from volcanoes during or following eruptions can range from flowing molten rock to flowing denser-than-air volcanic gas. But the bulk of volcanic flows involve two-phase mixtures of solid rock fragments and a fluid—either liquid water or gas composed of air ± volcanic gases. Deposits of the various particle–fluid flows derive their physical characteristics from mixture properties and from the particle–fluid interactions involved in both the transport and deposition of the mixtures. Interactions can differ substantially among emplacement processes. Flow types vary with the ratio of solid particles to fluid and with fluid composition. Volcanic “wet” flows involve variable mixtures of liquid water + volcanic rock particles (± ice fragments)—lahars, debris avalanches, and water floods. Their deposits exhibit properties differing only slightly from deposits emplaced by “dry” flows involving gas + volcanic rock particles and from deposits emplaced by flowing glacier ice. Until now, wet-flow deposits have not been systematically described in detail. Recognition of the characteristic features of any volcanic or volcaniclastic deposit can lead to identification of its emplacement process, which is critical for accurate volcanic hazard assessment.