Glacial lake outburst floods (GLOFs) from moraine-dammed lake failures represent a significant threat to inhabitants of high mountain areas across the globe. The first part of this paper summarises the causes and mechanisms of moraine-dammed lake failures through a review of the scientific literature and unpublished reports. There are eight main causes, of which five are characterised as dynamic and three as long-term, and these are associated with around twenty failure mechanisms. The dynamic causes are slope movements into the lake, earthquakes, flood waves from a lake situated upstream, blocking of underground outflow channels, and intensive rainfall or snowmelt. The long-term causes are the melting of buried ice, the impact of hydrostatic pressure, and the effect of time. These causes (triggers) and the consequent mechanisms of dam failure are described in detail. The second part compares the historical moraine-dammed lake failures within three regions between 1900 and 2009: the Cordillera Blanca of Peru, the North American Cordillera, and the Himalayas. It has been found that dynamic causes are around four times more common than long-term causes although significant regional differences have beenobserved. The most frequent causes in these regions were found to be slope movements in which the displaced material was dominated by solid-state water (ice falls, ice avalanches, and snow avalanches). The other causes tended to show distinct regional patterns while the temporal distribution of events also differs according to region. In the North American Cordillera and Himalayas moraine dam failures occur exclusively during the summer season while in the Cordillera Blanca they are more evenly distributed with the exception of the dry season. This reflects the general climatic setting of each of the study regions.