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Externally bonded fiber reinforced polymer (FRP) composites are an increasingly adopted technology for the renewal of existing concrete structures. In order to encourage the further use of these materials, a design code is needed that considers the inherent material variability of the composite, as well as the variations introduced during field manufacture and environmental exposure while in service. Load and Resistance Factor Design (LRFD) is a reliability-based design methodology that provides an ideal framework for these considerations and is compatible with existing trends in civil engineering design codes. This dissertation studies the application of LRFD to FRP strengthening schemes with an emphasis on wet layup, carbon fiber composites applied to reinforced concrete T-beam bridge girders. Models to describe variation in the existing structural materials and the structural loading are drawn from the literature. Techniques for reliability analysis are discussed, and existing work on externally bonded FRP reliability is surveyed. Stochastic variation in the FRP is characterized based on tensile testing of several sets of field- manufactured, wet layup composites. A general design procedure applicable to many different situations is proposed using a composite specific resistance factor to consider material variability, a set of Application Factors to account for deviations introduced through field manufacture, and an environment and service-life specific factor for FRP degradation. Preliminary resistance factors for design of FRP strengthening are calibrated over a range of design scenarios. FRP degradation is considered based on existing durability models, and continued degradation of the structure due to general corrosion of the reinforcing steel is included. The girders used for calibration are selected as representative examples from a sample of California bridge plans. The reliability has been evaluated using simulation and first-order reliability methods. An example of the proposed design procedure, using the calibrated resistance factors, is provided. The results of this work bring to light the many variables affecting the reliability of strengthened members and the need for continuing research to better describe these variables. Two variables of particular significance, requiring extensive further study, are the state of the existing structure when strengthening is applied and the loads acting on the structure