This report reviews the factors that trigger fish migration in the Mekong River and other tropical freshwater river systems. It aims to provide a basis for understanding the consequences of human intervention to the natural flow regime of the river system on fish migration and thereby on the fisheries of the Lower Mekong Basin. The report comprises two parts. It begins with a systematic review of the published information on migration triggers and cues in the Mekong River system and in other tropical rivers worldwide. This part includes a discussion of other associated issues, such as spawning triggers, which should also be taken into account when dealing with Mekong fish migrations and management. The second part of the report presents a quantitative analysis of the environmental factors that trigger the migration of Mekong species. This section is based on a merger of FishBase 2004 with the Mekong Fish Database, and covers the entire population of 768 Mekong species held within FishBase. The merged data set includes biological data and coded ecological information for all of these species. The results are supplemented by an analysis of data from a recently published study of migration patterns and hydrological triggers at the Khone Falls (Southern Lao PDR).