Beam diagnostics and instrumentation are an essential part of any kind of accelerator. There is a large variety of parameters to be measured for observation of particle beams with the precision required to tune, operate and improve the machine. Depending on the type of accelerator, for the same parameter the working principle of a monitor may strongly differ, and related to it also the requirements for accuracy. This report will mainly focus on electron beam diagnostic monitors presently in use at 4th generation light sources (single-pass free-electron lasers), and present the state-of-the-art diagnostic systems and concepts.