Remote sensing is the popular name for the measurement of physical quantities at a distance, usually by quantitative spectroscopic methods. Its application to the study of the Earth's atmosphere and surface from satellites is a rapidly growing field, with many applications, and new or improved techniques are constantly being developed. In this article we look at some of the basic principles and technical challenges involved, and at the benefits which are accruing in a number of research areas in earth science, with particular emphasis on the use of remote sensing to understand global change.