The durability of PV modules and their component materials to the stresses in the outdoor environment is the most important indicator of reliability,durability and safety of photovoltaic modules throughout their service life.Tedlar(R) polyvinyl fluoride films have been a key component in backsheet materials that have been in the field for over 25 years,and the performance in the field of these modules and components can be compared to durability tests and to other backsheet materials including PET.Test protocols are described including UV exposure based on solar irradiance in different climates and relevant albedo levels for exposure of the back of PV modules.The change in critical performance properties in durability tests including damp heat and UV is compared to backsheets extracted from fielded modules.Analysis of mechanical and chemical properties of the inner and outer layers of backsheets removed from fielded modules is examined.Impact of extended damp heat and UV to backsheet properties and module performance observed in the field is further quantified through measurement of mechanical,optical,electrical and permeability properties of the backsheet and power,electrical insulation and physical properties of the modules.Correlations between field performance and accelerated testing are demonstrated.The first results for sequential and combined stress testing are described and compared to field performance.A comparison of two large and diverse sets of modules from two different locations (EU and Japan) is discussed and power loss with field exposure period and the impact of backsheet is described.