Computational methods for calculating the molecular nonlinear optical (NLO) properties of molecules are reviewed, with an emphasis on clarifying the strengths and weaknesses of the various approaches. A brief introduction to the theory of NLO effects is provided, and a summary of the key experimental techniques for the determination of molecular first hyperpolarizabilities is included, with discussion of their advantages and disadvantages. Applications of semi-empirical methods and density functional theory in developing structure–quadratic NLO property relationships for organometallic complexes (and particularly metal alkynyl complexes) are reviewed.