Electron probe X-ray microanalysis (EPXRMA) is only one of a large number of techniques that are available for investigating the chemical composition of cells and tissues. These techniques use a wide variety of physical and chemical principles, and each has its own advantages and disadvantages (see Table B1). Some are non-microscopical and, at best, give information only at a rather coarse level of spatial resolution, although their chemical sensitivity may be very high. Among the microscopical techniques, many should be regarded as highly specialised, in the sense that the equipment is not easily available, and in some cases is not produced commercially. Other methods, such as histochemistry and autoradiography, may be regarded as routine, but do not, on the other hand, yield the same type of information as XRMA and other techniques. Indeed, while all the techniques listed in Table B1 can be used to study the chemical composition of cells or tissues, it seems that they are complementary rather than competing. For example, histochemical procedures can identify compounds, such as enzymes, which are not generally accessible by the other methods. As well as autoradiography, techniques such as SIMS (Thellier, Ripoll & Berry, 1991) and LAMMA, which can identify isotopes, are well adapted for the study of dynamic processes using tracers. Of the techniques considered in the following two chapters, particle-induced X-ray emission (PIXE) is most similar to EPXRMA, yet nevertheless shows important differences in specimen penetration, resolution and sensitivity.