Plasma diagnostics play an essential role in the development of fusion energy. This lecture series will start with a basic introduction to fusion and then explain various diagnostics from basic principles and how they are used to diagnose a hot plasma. The diagnostics covered will include magnetic probes, electric probes, refractive index measurements, Thomson scattering, electron cyclotron emission, heavy ion beam probe, charge exchange spectroscopy, x-ray imaging crystal spectrometer and bolometer. Finally, a brief introduction to tomographic techniques and applications will be given.