What are the reasons for Earth's climate shift from extremely warm "greenhouse" conditions during the Cretaceous to the present "icehouse" state? Various hypotheses draw on large-scale changes of poleward heat and moisture transports which were forced either by atmospheric circulation shifts or by tectonic reorganizations of ocean gateways such as Drake Passage. Alternatively, it has been supposed that enhanced burial of organic carbon and/or increased silicate weathering rates led to reduced atmospheric CO2 levels. None of these hypotheses is free of contradictions when compared with proxy data records. This indicates that our current understanding of the Cenozoic climate history is still limited and suggests that further forcing factors or feedbacks may have been effective, such as other greenhouse gases or the land vegetation. We invite contributions from climate and/or biogeochemical modelling as well as presentations of climate reconstructions by means of proxy data.