Plasma diagnostics are based on a multitude of different physical processes with wavelengths in the range from sub-nm to tens of cm. Many different techniques are being employed for measuring the spatial profile and evolution of various plasma parameters. Although a fair number of the techniques are already well established, plasma diagnostics is still a very challenging and vivid discipline. On the one hand this is caused by the always-continuing effort to attain a better spatial and temporal resolution, to reach higher accuracies and to measure with more spatial channels. On the other hand diagnostic techniques based on more subtle physical processes (than used in the routine diagnostics) are continuously being developed. This paper will give a brief introduction into the field of plasma diagnostics.