Renewable energy is an important part of the mitigation of climate change. Photovoltaic systems (PVs) are the fastest growing energy source. They use energy from the sun to produce electricity, not from burning fossil fuels. However, from a life cycle perspective, PV is not an emission-free energy source because greenhouse gas emissions (GHG) are produced, mainly during the manufacturing of components such as solar modules. Generally, these emissions are negligible compared to the ones produced from fossil fuels. Therefore, it is commonly assumed that only operation emissions are relevant. This thesis explores the GHG mitigation of optimized grid-connected PV systems. The results show that, in most cases, the mitigation of GHG emissions is considerably higher than the emissions. For example, in Cyprus the life cycle GHG mitigation potential is 18.6 t CO₂-eq / kWp, which exceeds by far the GHG emissions of PV systems which are between 0.46 t CO₂-eq / kWp and 1.11 t CO₂-eq / kWp. However, exceptions (where the mitigation potential is not enough to cover the GHG embodied emissions of the PV systems) are found if relatively high carbon PV modules are simulated in Sweden, France and Norway due to the mostly decarbonized grid in those countries. For example, in Sweden the life cycle GHG mitigation potential found is 0.63 t CO₂-eq / kWp, which can be higher or lower than the life cycle emissions from a PV system, depending mostly on the manufacturing conditions.