In this introduction section the basic categories of remote sensing are defined together with their specific applications and limitations. During the last and the present centuries amazing advances in environmental monitoring have happened due to an impressive breakthrough in satellite and sensor technologies. Traditionally remotely sensed products have been regarded as airborne or satellite images that quantify the amount of radiation reflected and/or emitted in specific channels of the electromagnetic spectrum. What we can really regard as remote sensing has become a wider concept and also includes missions that detect other properties of the environment than the radiative ones. Finally, it is explained how our focus book has been divided into its various chapters.