Much of the driving force behind laboratory plasma physics research comes from the prospect of using controlled thermonuclear fusion, based on either inertial or magnetic confinement, to provide economically significant amounts of power. A magnetically confined high-temperature plasma is highly complex and, to make progress in our understanding of the fundamental plasma physics, one must be able to make accurate measurements of internal plasma parameters. Only then can theoretical ideas be confronted quantitatively with experiment. The specific goals of fusion impose stringent constraints on temperature, density and confinement and so give rise to extra problems for plasma diagnosis. For example, the high central temperatures of thermonuclear plasmas exclude direct measurements using intrusive material probes which are used mainly to monitor the cool exterior plasma regions. A wide range of non-intrusive physical measurement techniques has therefore been developed for diagnosis, ranging from basic measurements with electromagnetic probes, spectroscopy, particle and photon scattering, charge-exchange spectroscopy to nuclear particle and photon measurements. Most methods involve gathering data which has been transformed in some way either by the data collection geometry or by instrumental limitation and so inverse transformation is needed to obtain local plasma physics quantities.