This paper reviews the field of microwave sensors. The field is broad and the applications numerous. This review will therefore only be able to present the broad outlines.

Firstly the historical perspective and the physical background are briefly described, and the general advantages and disadvantages are listed.

An overview of the various working principles of microwave sensors is given with a few examples of the applications mentioned. Important fields of applications and interesting examples are treated separately, starting with the measurement of moisture, which is the single most important field of applications in microwave sensors.

Applications in the petroleum industry are also mentioned, because they are relatively new, they play an exceptionally important economical role, and represent the field in which the author is currently working. Followed by some trends for the future.