Transcranial magnetic stimulation is a neuromodulation technique that can be used as a non-invasive method for treating various neurological disorders. The major principle of TMS is the Faraday’s law stating that time varying magnetic field induces electric field in nearby conductors. More specifically, magnetic field generated by TMS stimulator induces electric current in the conductive brain tissue. Numerous studies have been done to explore the effects of TMS, while the cellular and molecular mechanism underlying is not clear yet. In addition, coil design is also a popular topic as the geometry of coil is able to alter the induced electric field significantly. The work presented in this dissertation discusses the effects of TMS on neuronal cells in vitro and the computational simulations modelling the stimulations delivered to the head. A dopaminergic neuronal cell line is used to examine how TMS affects the proliferation of the cells in vitro. The effects of TMS promoting the proliferation of neuronal cells have been observed under two different cell culture environments. Furthermore, results of thousands of computational simulations were presented in this dissertation. Two coils were placed at three locations to investigate how the electric fields delivered to the cerebellum vary with coil geometry and coil position. Moreover, the intensity and focality of the electric fields generated in the brain by 16 commercial or novel coils were compared. All the coils were placed at the vertex and 9 of them were placed at the dorsolateral prefrontal cortex of the head. Importantly, 50 heterogeneous head models were used in these simulations for each coil and position to explore the role of anatomical variations in TMS.