The crystal structures of four charge-transfer complexes and two organo-metallic compounds have been determined. The various geometries encountered are related to structural isomerism and to physical properties. The charge-transfer complexes phenanthrene pyromellitic acid dianhydride (PMDA), fluorene PMDA, thianthrene PMDA and biphenyl trinitrofluorenone have planar molecules except for thianthrene which is bent. The molecule N-(2-hydroxyphenyl)salicylaldimine dimethyl tin(IV) is bipyramidal whilst the hexa-v-dithiocacodylato tetrazincsulphide isomers have a tetrahedral character. The concept of the crystalline state as a hierarchical system is developed. The physical basis, and electrical properties of charge-transfer complexes are reviewed, and the implications of structural features, and possible uses of these compounds in modern signal processing technology is discussed.