Organic second-order nonlinear optical (NLO) materials for electro-optical applications are gaining interest because of their superior properties in comparison with their inorganic counterparts. The design and selection of the materials, however, remains a mainly empirical undertaking due to the multi-level nature of this research. Therefore, in this project, both the molecular and supramolecular levels will be optimised for a promising category of compounds, in accordance with a fundamental theory which describes the NLO properties of organic solids. At the molecular level, information on the NLO potential of a molecule is obtained by calculating the hyperpolarizability. The most promising compounds are then synthesized, crystallized and characterized by single-crystal X-ray diffraction (XRD). At the supramolecular level, solid-state calculations under Periodic Boundary Conditions (PBC) are used in a crystal engineering approach to reveal the relative importance of the different supramolecular synthons that contribute to the formation of polar crystal structures: only crystals with polar space groups qualify since the presence of an inversion centre cancels out all secondary NLO effects. In addition, the nonlinear optical susceptibility, the macroscopic counterpart of the hyperpolarizability, can be estimated from the calculations, leading to a fast, pragmatic and cost-efficient selection of superior NLO materials.