Major loss of life caused by lahars (volcanic mudflows) in historical times has been largely restricted to the Circum-Pacific region and more particularly to Japan (>11,650 killed), Indonesia (>9,300 killed) and Central America (>1,300 killed). In addition to such losses of life, widespread damage may occur to buildings, bridges, communication networks and arable land. A review of the causal mechanisms of lahars, flow behaviour and protective measures, with selected case histories, is therefore appropriate to an understanding of this major geological hazard. The potentially most destructive lahars are those involving sudden release of very large quantities of water from crater lakes or from subglacial lakes. The Icelandic jokulhlaups, although not strictly lahars, give some idea of the huge discharges of water that can be released — ephemeral maximum discharge rates have been estimated up to 100,000 m3/sec, or temporarily equivalent to the flow of the River Amazon. Other potentially destructive lahars are those resulting from pyroclastic flows becoming admixed with running or ponded waters. Of more common but less devastating occurrence are lahars generated by heavy rainfall on the slopes of volcanoes, more particularly on recently ejected pyroclastics. Historical lahar disasters of this type occur most frequently in tropical regions. Other initiating mechanisms include melting of snow and ice directly accompanying eruptions, earthquake triggered collapse, phreatic explosions and directed blasts. Historical lahars generated by these mechanisms have not been responsible for any considerable loss of life, with the exception of the Shimbara Catastrophe in Japan where a lahar entered the sea producing tsunamis. Upon initiation of a lahar, mud, sand and gravel combine with available water to form a high bulk density (>1,400 kg/m3) flow. In some lahars the flow behaviour may approximate to a Newtonian liquid, whilst in others a high concentration Non-Newtonian liquid is formed with the capability of transporting very large clasts which may each weigh over 200 tonnes. The formation of a laminar boundary layer at the base of the flow is responsible for a low friction factor that enables some lahars to travel very large distances (>100 km). It also explains how lahar deposits often overlie completely undisturbed yet easliy erodible materials. This boundary layer can often be identified in many lahar deposits by a fine-grained layer at the base. The continuous phase of such lahars exhibits strength which retards the sinking of boulders and is responsible for the unsupported framework and poor sorting of lahar deposits. Protective measures against loss of life and damage to property are discussed with particular reference to case histories in Indonesia and New Zealand. Indonesian measures have included siphoning water from the crater lake of Mt. Kelut, effective warning systems, and preparation of maps showing regions that may be destroyed by lahars. In New Zealand, two principal centres of Post-glacial lahar activity are Mt. Ruapehu and Mt. Egmont. Since 1861 A.D. eight lahar episodes have been generated from the crater-lake on Mt. Ruapehu, the 1953 lahar being responsible for the ”Tangiwai Disaster”, when 151 persons were killed. Existing and future protective measures against Mt. Ruapehu lahars are discussed. Mt. Egmont has a long record of Post-glacial lahar activity. The causal mechanism of some Egmont lahars has been heavy rains, but the existence of a former crater lake in the summit area cannot be discounted. Based on detailed geological, pedological and botanical investigations a geological hazards map of the Mt. Egmont region has been prepared.