This thesis explores the “crystal engineering” of the solid-state behaviour of a series of alkali metal TCNQ salts. This can exhibit a variety of electronic and magnetic properties, depending on the solid-state architecture. TCNQ is a good one-electron acceptor and the resulting radical anion salt is quite stable. The architectural behaviour of TCNQ salts is very dependent on the nature of the counter-cation and the stoichiometry of the material. In the present study, the effect of ionophore-encapsulation of the cation (M = Li, Na, K, Rb and Cs) has been explored using single crystal X-ray diffraction, IR, Raman, EPR and pressed discs conductivity measurements. In addition, the effect of changing the ionophore:metal cation ratio and the presence of additional TCNQ0 has been investigated. 25 new crystal structures have been obtained and analysed in detail and this has grown new insight into the impact of the effect of controlling ion pair interactions through ionophore complexation and of steric factors on the nature of TCNQ assemblies adopted. A range of solid-state motifs have been observed including some novel solid-state behaviour. In addition, the solid-state behaviour of two hydrated lanthanide TCNQ salts has been investigated.