In today's electrical measurement technology, complex electronic measuring devices are used, which predominantly contain facilities for electronic signal processing. The measuring instrument in the narrower sense, which converts the electrical quantity of voltage into a pointer deflection or a digital display, represents only a small part of the electronic measuring device. Therefore, to fully utilize the capabilities of an electronic measuring device, no fundamental measurement errors should be made, and the measurement results must be interpreted correctly. The device concept and the resulting signal processing must be understood. Thus, measuring instruments and their operation are essential contents of the book. The description of the predominantly manufacturer-independent device concepts, which reflect the current state of high-frequency measurement technology, is based on devices from Hewlett & Packard and Philips.