The Antarctic Bottom Water (AABW), which ventilates most of the deepest basins of the world ocean, has shown widespread warming in recent decades that contributes inter alia to the global thermosteric sea level rise. More than half of the AABW is supplied by outflow from the Weddell Sea where it attains its characteristics, in particular its high density, by heat loss of its source water masses to the atmosphere and to the fringing ice shelves and by influx of locally formed water masses such as the high-salinity shelf water shaped by brine release from freezing sea ice. Dense shelf waters cascade down the continental slope along the southern and western rim of the Weddell Sea, mix with the modified source waters and thus replenish the Weddell Sea Bottom (WSBW) and Deep Water (WSDW), precursor of the AABW. The major source water mass, hence conduit of heat for the interior Weddell Sea, is the Circumpolar Deep Water (CDW) advected from the Antarctic Circumpolar Current (ACC) with the southward oriented eastern limb of the Weddell Gyre. During its advection, the CDW is sucked upward by divergence in the large-scale mean wind-field to about 200 m in the central Weddell Sea, where it is termed Warm Deep Water (WDW). Progress in the understanding of this twisted transformation of water masses is obtained from a combination of three different observational approaches. First, a gridded data set of upper-ocean hydrographic properties in the Weddell Gyre derived by objective mapping of under-ice Argo float profiles; this data set reveals for instance the WDW inflow, its transport with the Weddell Gyre, and the erosion of its core thermohaline properties during circulation. Second, multi-annual time series obtained from repeat deep CTD stations, which demonstrate a significant, approximate linear warming trend in the Weddell Sea deep water masses below 700 m depth of roughly 2 mK per annum during the past 25 years. Third, long-term mooring records of velocity and temperature in the Weddell outflow regime on the northwestern continental slope, which reveal sizable intra-seasonal and seasonal fluctuations in the WSBW plume, and a hardly significant multi-annual trend which indicates rather a cooling than a warming and therefore differs from the significant warming seen in the deep waters of the Weddell interior.