 The sources and types of observational data employed in climate science are tremendously varied. Data are collected not only at land-based stations, but also on ships and buoys in the ocean, on airplanes, on satellites that orbit the earth, by drilling into ancient ice at earth’s poles, by examining tree rings and ocean sediments, and in other ways. Many challenges arise as climate scientists attempt to use these varied data to answer questions about climate and climate change. These challenges stem both from the character of the data—they are gappy in space and time, and they are obtained from instruments and other sources that have limited lifespans and that vary in quality and resolution—and from the nature of the questions that climate scientists seek to address, which include questions about long-term changes on a global scale.