Over the 4.6 billion year history of our planet, five major ice ages have occurred in addition to other periods of climate cooling. The specific cause for these events is most likely multifactorial, including alterations in solar radiation (related to both solar output and subtle changes in the Earth's orbit), fluctuating ocean currents (primarily secondary to plate tectonics and continental drift) and chemical changes in the Earth's atmosphere as a consequence of vegetation patterns, volcanic activity, asteroid impacts and both the production and absorption of greenhouse gases (primarily methane and carbon dioxide).
The first major Ice Age, known as the Huronian, developed about 2.4 billion years ago, while the Cryogenian began 850 million years ago and may have nearly enveloped the entire planet in a sheet of ice; both of these early Ice Ages lasted more than 200 million years. The Andean-Saharan Ice Age stretched from 460-420 million years ago (MYA) while the Karoo Ice Age began 360 MYA and lasted 100 million years. The fifth major Ice Age, known as the Quaternary Glaciation, began about 2.5 MYA and, most climatologists believe, continues today (though we are currently in one of its warm interglacial periods that began 10-15,000 years ago). Periods of climate cooling also occurred during the Oligocene, about 35 MYA and the Miocene, some 15 MYA, each lasting 10 million years or more; it was during the Oligocene cooling that the Antarctic ice sheet began to form, now harboring 60% of the freshwater on Earth.
It should be emphasized that Ice Ages are characterized by prolonged periods of climate cooling and glaciation (both continental and cordilleran) but are broken by relatively warm interglacial periods such as the one in which we live. During these warm periods, glaciers retreat and sea levels rise; conversely, as the climate cools, glaciation expands, sea levels fall and continental land masses expand (including the formation of land bridges). The current warming cycle is progressing at an exceptionally rapid rate and most climatologists are convinced that human activity (including fossil fuel utilization and deforestation) is responsible. There is little doubt, however, that, within another 20,000 years or so, the climate will begin to cool and the ice will return.