Solar photovoltaics (PV) is one of the most competitive renewable energy technologies in order to meet the increasing global energy demand and decrease CO2 emissions by competing effectively with fossil fuels. One of the important applications of PV energy is building integrated photovoltaic (BIPV) systems, which transforms building envelopes into power plants while keeping the building properties such as thermal isolation, water-tightness, shading etc. A massive deployment of PV in buildings - where about one third of primary energy consumption takes place in developed countries - needs to be favored in order to avoid conflicts of PV with other land use (e.g. agriculture, forestry). This is particularly significant in countries like Switzerland. PV manufacturers provide 25 years of warranty for the PV modules. However, there is a trend going towards 30-35+ years of lifetime for properly produced modules. BIPV modules could experience somehow harsher operating conditions compared to field deployed PV modules. Nevertheless, BIPV products are expected to have long service lifetimes since other building components generally have lifetimes targeting 40 years. By improving the reliability and durability of BIPV products, thus its lifetime, BIPV could be more appealing and cost-effective for building owners, installers and architects. The main aim in this PhD work is to assess and improve the reliability and long-term performance of BIPV modules and systems. This will be done by: 1. Analyzing long-term performance data from a statistically representative number of BIPV systems and mock-up structures in order to assess the impact of specific operating conditions on degradation rates of BIPV modules 2. Performing site surveys on selected BIPV plants to understand the degradation mechanisms related to operating condition 3. Designing accelerated aging tests in order to address specific failure modes that BIPV modules experience