This report presents a draft methodology for the assessment of transboundary lake basins and forms the basis for a follow up full-sized project. Lakes, along with rivers, groundwater, large marine ecosystems, and open ocean make up the five TWAP waterbody types. While interlinkages to other waterbody types is a recurring theme in this report and in TWAP itself, we look explicitly at lakes here, or more correctly, at lakes and their drainage basins.