The rapidly developing field of organic electronics has become a particularly important branch in modern solid state physics. Organic semiconductors exhibit interesting physical properties which offer the possibility to design devices with novel structures and functionality such as photovoltaic cells, chemical sensors, and flat panel displays. The performance of organic electronic devices crucially depends on the material purity and molecular ordering. Therefore, careful experimental characterization and controlled film growth are of prime interest. A sound theoretical background capable of describing and analyzing the materials properties is equally important for the advance of organic electronics. It is a hot topic of a ongoing research and the goal of this thesis. Density functional theory (DFT) is one of the most powerful and popular theoretical ab-initio methods for describing structural and electronic properties of a vast class of materials. The recently developed van der Waals density functional (vdW-DF) has allowed successful handling of weakly bound systems such as crystals formed by organic molecules or molecules physisorbed on metal surfaces. An efficient implementation of vdW-DF is hence a prerequisite for tackling cohesive properties of organic semiconductors, and represents a central aspect of this work. This approach is applied for the investigation of cohesive and surface energies of three classes of molecular crystals based on small $\pi$-conjugated molecules. The results are found to be in excellent agreement with reported experimental data for cohesive energies. This is particularly important since surface energies are experimentally difficult to access. The vdW-DF computer code developed within this thesis has also enabled the study of even more complex situations as the adsorption of organic molecules on metal surfaces. Hence for several examples, including thiophenes on clean and oxygen terminated Cu, as well as PTCDA on Cu, Ag, and Au, the role of the nonlocal correlations could be investigated in detail. Another emphasis of the thesis is on the structure solution of organic molecular solids which show a strong tendency to form polymorphic phases. The molecular arrangements in organic thin films are influenced by the interaction with substrates and lead to substrate-induced crystal structures which can be rather different from the observed bulk structures. A combined experimental and theoretical study is proposed and applied to pentacene and cyano-quaterphenylene to solve the crystal structure of their thin film phases.