Nonlinear optical (NLO) chromophores containing transition metals have been investigated widely because they offer fascinating combinations of physical properties and great scope for molecular engineering. This review summarises our recent studies with NLO metallochromophores, focused largely on complex salts of iron or ruthenium. Highlights of this work include: (i) the extension of our solution-state studies with Ru II ammine chromophores to the first demonstration of redox-induced switching of bulk NLO behaviour; (ii) heterobimetallic ligand-bridged Ru II /Re I complexes for which Stark spectroscopy provides useful data; (iii) Fe II cyanide complexes that show very strongly medium-dependent and pH-switchable NLO responses; (iv) Fe II or Ru II tris(2,2′-bipyridyl) derivatives that combine very large quadratic and cubic NLO properties; (v) unusual ferrocenyl “diquat” species that show multiple redox states, 2D NLO responses and a polar crystal form in one case. These studies reveal extensive and logical correlations between charge-transfer absorption, electrochemical and NLO behaviour, with the experimental observations being supported by theoretical calculations in several instances.