Fluorescence techniques have a wide range of applicability. In non-destructive testing they are used for the detection of X-rays, and also in the examination of metals. Magnetic materials can be tested by the fluorescent magnetic particle method, and the surface of any metal can be inspected for cracks using the fluorescent penetrant method. The main properties of the luminescent centre are outlined with a view to obtaining a deeper understanding of the processes involved, and to applying them to further types of problem. Examples of fluorescent substances include pure and impurity-activated solids, both organic and inorganic. Metals are non-fluorescent, but materials which can be directly studied by fluorescence include some glasses, ceramics and textiles.