The corrosion of steel in concrete has been identified as one of the most important causes of deterioration and structural deficiency. A relatively recent solution, receiving global attention, is the use of fibre reinforced polymer (FRP) composites as a cost effective alternative to traditional construction materials and techniques. The low cost-to-performance advantage of glass fibre reinforced polymers (GFRP) is the driving force behind its world wide use and acceptance. GFRP materials are attributed to being of high strength, light weight, non corrosive and non conductive. Unfortunately, in some special conditions, such as in high alkalinity environment, the long-term performance of the GFRP is still unresolved question. Strength of the glass fibres and resin matrix, the two constituents of the GFRP materials, can decrease when subjected to a wet alkaline environment. This book summarizes the effects of various conditions on the durability of FRP composites for use as internal reinforcement for concrete structures.