Two important microwave remote sensors are the radar and the radiometer. There have been a number of books written on various aspects of radar, but there have been only a few written on microwave radiometers, especially on subjects of how to design and build radiometer systems. This book, which is the second edition of a book originally published in 1989, attempts to fill this void. The background for this book is many years of work with radiometer systems including design and manufacture of airborne imaging radiometer systems, laboratory as well as airborne field experiments with the systems, and design of future spaceborne imagers. This book would not have been possible without the support and encouragement of several colleagues. Søren Nørvang Madsen, who is working with synthetic aperture radar systems, and, before him, Finn søndergaard have both contributed much to the work with radiometer systems through many fruitful discussions and via joint airborne experiments. Much support was received from Professor P. Gudmandsen, especially in the initial phases. The value of fruitful interaction with Ph.D. students like Brian Laursen and Sten Søbjærg also cannot be overestimated. Finally, the partnership with Calvin Swift and several of his students in the development of the synthetic aperture radiometer, ESTAR, provided invaluable learning that helped make this book possible.