The polar ice sheets hold one of Earth’s great sedimentary records. By drilling ice cores from Greenland and Antarctica, we can obtain ice that fell as snow, extending back so far 800,000 years in Antarctica and over 125,000 years in Greenland. Ice cores contain information about climate and numerous other environmental parameters; crucially the air bubbles trapped in the ice give access to the past composition of the atmosphere, including the greenhouse gas concentrations. In this talk I will first discuss the strengths and weaknesses of ice cores, and then demonstrate how ice cores are collected. I will then present a few examples of the knowledge we have gained from ice cores – about greenhouse gases, about glacial/interglacial cycles, and about rapid climate changes most likely induced by changes in ocean heat transport. I will present an example of how we are trying to use ice cores to reconstruct the past extent of sea ice, a critical component of climate feedbacks. Finally I will discuss prospects for obtaining even older ice in the future.