Vast, majestic, and often stunningly beautiful, glaciers lock up some 10% of the world's freshwater. These great bodies of ice play an important part in the Earth system, carving landscapes and influencing climate on regional and hemispheric scales, as well as having a significant impact on global sea level. Throughout time,the Earth has experienced various major glaciations in its deep history, long before the ice ages of the Quaternary, and the observed effects of climate change on glaciers have recently brought them to the forefront of public attention. This Very Short Introduction offers an overview of glaciers and ice sheets as systems, considering the role of geomorphology and sedimentology in studying them, and their impacts on our planet in terms of erosional and depositional processes. Looking at our glaciers today, and their ongoing processes, David Evans considers the extent to which we can use this knowledge in reconstructing and interpreting ancient glacial landscapes.