"The revolutionary potential of nanoscience lies in the ability of designing new materials and functionalities for specific applications taking advantages of quantum nature of electrons. After the breakthrough discovery of graphene, the recent years have witnessed an explosive interest in two-dimensional Transition Metal Dichalcogenides nanosheets - diselenides and disulfides of transition metals, like MoS2, WS2, TiS2 and WSe2 - due their high technological potential for renewable energies, nanoelectronics and nanocatalysis. Major advances in this field heavily depend on the understanding of the electronic excitations sustained by these nanostructures under irradiation (by light, electron beams, synchrotrons or ultra-fast lasers) and on the ability to link the local chemical, electronic and structural modifications to changes in their macroscopic optical behavior. This project aims to provide a deep insight (through theory, simulation and experiments) to the electronic and optical properties of semiconducting Transition Metal Dichalcogenides nanosheets. By applying innovative computational modelling ab-initio techniques - within the framework of Time Dependent Density Functional Theory - the electronic structure and optical response of such systems will be investigated from the atomic scale up and then compared to experiments (i.e. electron energy loss spectroscopies). One key objective will focus on quantum confinement effects, evaluating the change in the electronic spectra when the size of the system is reduced. A second and fundamental aim is to investigate chemical modifications, e.g. the insertion of dopant atoms or molecules into the layered structure of the material. Due its high fundamental scientific content, enormous technological potential and strong multidisciplinary character, the present proposal will provide a significant step towards the understanding of optoelectronic properties of nanostructures through theory, simulation and material designing."