The state and dynamics of tundra wetlands under climate variability in Siberian Polar Regions is the topic of a joint German-Russian research project that started in 2010. Two expeditions focusing, among other topics, on limnological and paleographic characteristics of thermokarst lakes were carried out in the Indigirka River floodplain (Kytalyk, 70°N 147°E) and in the Kolyma Delta (Pokhodsk, 68°N, 161°E) in 2011 and 2012. A major objective was to explore the ages and geneses of periglacial landscapes by studying the origin, location, and shapes of eleven different thermokarst lakes. Pursuing these objectives, scientists carried out coring of ground deposits. Collected sediment cores will be analyzed for TOC, TIC, TN, TS contents, grain size composition and stable isotope ratios (δD, δ18O, δ13C) and geochronology in order reconstruct lake dynamics and to establish sedimentation rates in the different stages of lakes evolution. To distinguish different types of thermokarst lakes, hydrochemical data (e.g. pH, oxygen concentration, acidity, alkalinity, main ions) as well as temperature and electrical conductivity were measured. Based on morphometric (lake size, lake area, shoreline shape, lake orientation) and bathymetric data, different lake types were classified with respect to their form e.g. round, triangular) and orientation. Thermokarst lakes in thermokarst depressions, on the higher plain of the Khalerchinskaya Tundra (Kolyma Lowland) and lakes of floodplain genesis were classified. In the Yedoma ice complex landscape of the Indigirka River region (Kytalyk), the border of modern thermokarst lakes seems to be parallel to the border of old dry lakes basins (alas depressions). The shape of modern lakes remains round and has an average depth of 1.3 to 1.5 m. The surrounding territory is dominated by polygonal structures, including numerous small polygonal ponds. Lakes in the Kolyma Lowland are deeper (in average 2.2 to 2.5 m) and have a triangular shape and a north-eastern orientation. The Kolyma Lowland lake density is significantly higher than in the Indigirka Lowland. Bathymetric measurements allow the construction of 3D-models of lakes useful for differentiation. Physico-chemical characteristics of the studied lakes together with climatic data obtained from weather stations (e.g. air temperature, precipitation and solar radiation) helps to elucidate the modern condition of thermokarst lake formation. The project was implemented with financial support from German Science Foundation (DFG), the Russian Foundation for Basic Research (RFBR 11-04-91332) and the German Academic Exchange Service (DAAD).