Antarctic sea-ice is a critical component of the climate system, an enhancer of glacial climate and, as recently hypothesised by modelling studies, a potential driver of the millennial scale climate variability that dominated the last glacial cycle (LGC). Unfortunately a severe lack of glacial sea-ice records from the Southern Ocean has so far prevented the testing of this hypothesis with field data. In this thesis, I present detailed reconstructions of Antarctic sea-ice and ocean conditions derived from diatom assemblages and the first application of highly branched isoprenoid biomarkers to glacial sediments. These sea-ice sensitive proxies were measured in high-resolution, glacial sediment cores from the Scotia Sea (Southwest Atlantic, West Antarctica) and the Adelie Land Coast (Australia Antarctica Basin, East Antarctica). Good chronological control for the past 50 kyrs was achieved through the correlation of geochemical tracers with an oxygen isotope stack, a combination of biostratigraphic datums and relative geomagnetic palaeointensity data, and the identification of the Laschamp geomagnetic excursion at its most southerly site to date. These records permit a critical assessment of the contemporaneous nature of the regional extent, duration and seasonality of summer and winter sea-ice in West and East Antarctica during the LGC, and further afford an opportunity to determine validity of the proposed role played by Antarctic sea-ice in millennial-scale climate change through its influence on oceanography and climate. Results show that the environmental response to climate perturbations in West and East Antarctica was heterogeneous between 46.9 cal ka B.P. and 25 cal ka B.P. and broadly homogenous between ''25 cal ka B.P. and deglaciation. This study builds on existing Last Glacial Maximum (LGM) sea-ice reconstructions and shows greater summer sea-ice expansion in the Scotia Sea than previously recognised, a reduced maximum winter sea-ice extent along the Adelie Land Coast, a circum-Antarctic sea-ice maximum earlier than the LGM, decreased sea-ice seasonality in the Scotia Sea prior to maximum conditions, and an extensive period of extended sea-ice seasonality after maximum conditions. Further, this investigation has revealed close relationships between these new glacial reconstructions of Antarctic sea-ice and Antarctic Isotope Maxima, Dansgaard/Oeschger events, atmospheric CO2 variability and deep-water formation, confirming the likely importance of Antarctic sea-ice in the propagation of global millennial-scale climate change during the LGC.