New particles in strong fields?

The recent search for new particles in the MeV mass range is reviewed. The most relevant experiments, most notably the GSI positron experiments, are described and their results are combined to restrict the properties of the hypothetical particles. The theoretical analyses of these and a large number of other experiments are presented in detail. Finally some special candidates for new particles such as the axion and localised complex vacuum excitations are discussed. The author discusses a number of ideas and suggestions which merit further investigation. With respect to the GSI experiments it is concluded that it is very unlikely that the observed e⁺-e^- coincidences indicate the existence of new particles.