The field of molecular electronics is continuously striving towards ever smaller electronic devices. To achieve this an intricate understanding of the electrical properties of molecules connected to metallic leads is vital. This thesis derives a general analytical formula, that describes the transmission through a single molecule, with the advantage that it can be evaluated on a pocket calculator. The tight binding model is applied to a multi-branch structure to derive the probability of transmission through the system, the results of which are tested against ab-initio simulations, which utilise a combination of Density Functional Theory (DFT) and the Green's function formalism to compute the transmission coefficient. The results highlight a range of interference effects in single and multi-branched structures and show a close agreement to tight binding formula. Further to this a study of the effects of an external electric field applied to a molecular junction is presented, where the aim is to construct a molecular switch based upon 7f-7f and van der Waals (vdW) interactions. Polycyclic Aromatic Hydrocarbons (PAH) are found to adsorb to each other, whereby the barriers to rotation of the adsorbed molecule relative to the base molecule can be influenced by an external electric field, leading to stable states that can be identified from the energy minimum of the system. A study of P AH systems is presented leading to a proposed molecular switch based upon applying an electric field to control the energy minimum.