An introduction to the Instrumentation, analysis of spectra and theory of electron spin resonance (e .s .r.) solids is given. The preparation and fabrication of doped zinc sulphide direct-current electroluminescent (d.c.el.) display screens is discussed. The e.s.r. technique was used to determine the electronic structure of the crystal defects and impurities in these materials. The nature and role of the "copper-coating" in such screens was elucidated using e.s.r. The mechanisms of "forming", "ageing" and d.c.el. are discussed in the light of the e.s.r., optical and electrical measurements. The preparation and operation of zinc oxide acousto- electric oscillators is discussed. An e.s.r. study of the materials used in such devices was undertaken. A new assignation of the ZnO-B centre is proposed and the lithium oxide centre was produced in lithium doped zinc oxide by simply passing an electric field through the crystal at 77 K. The e.s.r. technique was used to detect the 2S- state lead, tin and mercury ions in various diamagnetic media. A linear relationship between the magnitude of the 207 Pb 3+ ion's A-value and the partial ionic character of its environment was found. The HgH2+, HgOC2H52+, HgOH2+, Hg2+/3+ and PbCH32+ ions were identified using e.s.r. and some other unidentified species are discussed. The Appendices outline the calculation of the eigenvalues of systems when the "spin wave functions" are mixed.