This contribution presents an overview of the principal classes of second-order nonlinear optical (NLO) transition metal (organometallic and coordination) complexes and illustrates how organometallic and coordination chemistry can offer a very large variety of NLO structures in relation to the metal nd configuration, oxidation state, spin state, etc. Moreover, metal complexes can satisfy the very different demands of second-order NLO materials such as switchable, tuneable, and multidimensional NLO properties, depending on the subtle interplay of structure–property relationships. Recent advances and perspectives in metal complexes as NLO materials are also presented.