System monitors record inner states of computing systems. They are required for the debugging of computer systems as well as for the measurement of performance and they are used for the verification of system models, too. This paper first discusses the area of application of system monitors, and afterwards it introduces measurement-principles of the different monitor types: Software monitors, that either analyze the account-log, or that are available as event-driven monitors, as samplers or as profiling-monitors. Hardware monitors, using the measurement-principles logic-analyzer, events, sampling, and constructing classes of states. Hybrid monitors, which use the measurement-principles of hardware-monitors on the hardware part, but differ by the software part that generates the signals to be measured.