Antarctic sea ice behaves differently to its Arctic counterpart in many aspects, due to its thicker snow cover and to different meteorological and oceanographic boundary conditions. Most strikingly, the upper portions of Antarctic sea ice often possess extended horizontal gaps or porous layers filled with brine or sea water, in particular in summer. Often, these layers are inhabited by high standing stocks of active biological sea ice communities. These are responsible for the high primary productivity associated with Antarctic sea ice, and server as a feeding ground for larger animals when the ice melts.Processes are reviewed which are responsible for the formation of those gap layers. While some hypotheses point to the importance of absorption of solar radiation by algae to generate those gaps by melting, it is shown that there are a number of physical processes which are able to generate those gaps without any biology at all. Among those processes are flood-freeze cycles during winter, and freeze-melt cycles during summer, associated with a general warming of the ice during summer and with the formation of superimposed ice. Gap layer formation is supported by the survival of an intact snow layer on Antarctic sea ice in summer.New results are shown from the Ice Station Polarstern (ISPOL) drift camp carried out by the German research vessel Polarstern between November 2004 and January 2005 in the Western Weddell Sea. The main goal of that experiment was the observation of physical-biological properties and processes during the onset of summer melt.