AbstractFusion research relies heavily on plasma diagnostics in order to operate safely, to explore and optimise the performance of a fusion experiment and to study in detail the plasma physics. In recent years, there have been many advances in plasma diagnostics, not only as a result of the development of new and innovative techniques, but also by the improvement and careful application of well-established methods. A magnetically-confined plasma has many degrees of freedom and is influenced by many boundary conditions. Consequently, to fully characterise a fusion plasma the measurement of a correspondingly large number of plasma parameters is required. Some parameters, such as the magnetic field, current, temperature and density, are fairly obvious requirements, but some are more subtle or complex. Many of these parameters cover very wide dynamic ranges, either in different regions of the plasma at the same time or at different times during the evolution of the plasma. Obvious examples are plasma temperatures, which range from a few electron volts (a mere ten thousand degrees Celsius) in the divertor to tens of kilo-electron volts (a few hundred million degrees) in the plasma core and neutron yields, which range over many orders of magnitude. Usually, such wide ranges cannot be measured with a single instrument or measurement method. Consequently, many parameters require several different diagnostic instruments to cover their whole dynamic range. Moreover, independent methods of measuring the most important parameters, particularly those used for feed back control, are desirable for reliability and to resolve uncertainties in interpretation or interference from other effects. JET commenced operation in 1983 with about 20 diagnostic systems and now has about 70. Even so, some important parameters are not completely diagnosed.Measurements in fusion plasmas utilise a wide variety of techniques. Some methods are passive and use the detection of particles or radiation emitted spontaneously from the plasma, other methods use active probing of the plasma with beams of particles or electromagnetic radiation from an external source. Diagnostics based on electromagnetic radiation are particularly important; fusion plasmas emit electromagnetic radiation over a very wide spectral range, from high energy gamma rays through hard and soft X-rays, ultra-violet, visible, infra-red to microwave radiation. Each wavelength range conveys information about specific aspects of the plasma and coverage of the whole range is necessary.It is impossible in the space of a short paper to cover the wide range of recent diagnostic development. This paper will discuss some of the general principles and requirements. A few examples will be used to illustrate the present state of the art. We will briefly consider the future requirements and challenges of designing diagnostics for ITER and fusion reactors.