An outstanding question in the earth sciences is the fundamental reason for the initiation and the demise of large continental glaciations, referred to as "icehouse" periods in Earth history. In this paper, we propose a unifying explanation for the three main icehouse periods during the Phanerozoic (the last 545 million years). These glaciations occurred at: (1) ~34-0 Ma (earliest Oligocene to Present; (2) ~354-269 Ma (early Carboniferous to early Permian), and (3) ~444-443 Ma (late Ordovician Hirnantian stage of the Ashgill). The duration of the youngest two glaciations is measured in tens of million years, whereas the Ordovician glaciation was anomalous in that, although very intense compared with the Pleistocene glaciations, it lasted less than a million years, and occurred during what is believed to be an essentially greenhouse period in Earth history. Despite these differences in their duration, we propose that all these glaciations were initiated and terminated as a result of plate-tectonic processes. These placed continents in polar latitudes or removed them to lower latitudes, created the necessary topography, and opened and closed oceanic gateways essential for inducing the oceanic circulation needed for widespread ice formation. As is the case for Neogene glaciations, oceanic gateways are considered to be critical for the initiation of Palaeozoic glaciations.