The study of the earth's climate requires reliable global data sets to validate numerical simulation models and to identify regional and global fluctuations and trends. This book presents ways to obtain such data from space-borne and ground-based measurements, both passive and active, over the entire electromagnetic spectrum. It describes the basics of such methods together with the most recent advancements and spans the field from clouds and the planetary radiation budget to surface processes and ocean properties. Each subject is backed by extensive reference lists to enable readers to probe more deeply.