In these lecture notes a general description of diagnostic techniques for investigating laser-matter interactions will be given. Focus will be on laser-gas interactions relevant to plasma acceleration and laser-solid interactions of interest for Inertial Confinement Fusion studies. A detailed discussion on the temporal and spatial features of laser pulses amplified by the Chirped Pulse Amplification (CPA) technique will be given, with a special attention to the properties that play an important role in experiments, including pulse duration, contrast, pre-pulses and focusing configurations. Plasma formation and plasma density evolution by optical interferometry techniques will be presented along with the techniques for recovering information when optical interferometry with ultrashort pulses is adopted. Further, X-ray imaging and spectroscopy of plasmas will be introduced with attention to novel monochromatic imaging technique applied to plasmas generated by ultrashort laser pulses for investigation of fast electron transport in solids.