Over a period of 14 years, and as part of the Polish Archaeological Record research project, the author directed a field survey of the whole of the Lega river basin (NE Poland) – an area of over 850 km2. As a result of the survey of this hitherto poorly investigated area, almost 1100 archaeological sites were discovered, 748 of which contained lithic materials. The most interesting sites were initially studied by detailed mapping of surface-find distribution and later by excavation. A complete study of the whole drainage basin was very important for settlement research. The large number of the surface lithic assemblages enables reliable statistical analyses and accurate cartographic studies supported by palaeo-environmental reconstructions. Lithic assemblages were analysed from the point of production techniques and typology, dispelling some doubts about chronological-cultural affiliations of flint inventories from north-east Poland and neighbouring countries, but also raising new questions. It was possible to observe clear and changing settlement preferences in the late Palaeolithic and Mesolithic in the zone of late Glacial lakes and their adjacent outwash territories to the south. Six concentrations of sites were discovered, associated with communication routes and zones of economic activity.