The purpose of this paper is to provide a range of information which may be useful during dialogue to establish appropriate river flow regimes in the Sekong River in Viet Nam, Lao PDR and Cambodia when large hydropower projects are constructed and operated. It is designed to provide baseline or background information to assist in the process. The first part of the sourcebook starts by putting the Sekong River into context as one of the most significant large tributaries of the Mekong, with some information on the basic hydrology of the Sekong provided. The second part is based upon a description of the Sekong using analysis of its features as observed through Google Earth, to help distinguish and classify the different reaches of the river and their geomorphological and ecological features so that the effects of changing flow regimes can be assessed. Other chapters include information on fish biodiversity and basic information on the human populations and settlements along the river. The final chapters consider the existing and proposed hydropower projects and status of irrigation projects.