Rapid developments in the field of organic solar cells have been driven by this technology’s potentially advantageous traits: the environmentally friendly, low-cost generation of energy with the possibility of large area manufacturing of flexible, lightweight, semi-transparent devices, with predicted low energy payback times. Major step changes leading to vastly improved devices with ever-increasing performance have been achieved through new insights into materials design and an improved understanding of the often complex microstructure and phase morphology of organic solar cell systems. This chapter summarises the advances in synthesis, concentrating on the relevant structure/property relations and how the chemical structure affects processing and the microstructure. This is followed by a detailed discussion of classical materials science approaches that assist in gaining insights into complex materials systems, such as organic solar cell blends from the molecular to the micrometre scale, with a focus on polymer-based systems and how to apply this knowledge to future larger area processing of organic photovoltaic architectures.