This talk presents a range of materials by design methods that are being employed to systematically discover new classes of organic materials for optoelectronic applications. These methods include large-scale data-mining, using empirical methods (embracing discrete mathematics) and high-throughput quantum-chemical calculations. Two case studies illustrate the predictive success of this work: the materials discovery of high-performance organic (1) non-linear optical materials, and (2) dyes for dye-sensitized solar cells.