||Using fiber-reinforced plastic (FRP) to strengthen or retrofit existing reinforced concrete members has become more and more popular in the last couple of decades. For concrete beams retrofitted with externally bonded FRP, failure can occur due to flexural or shear crack initiated FRP debonding from the concrete substrate. In this thesis both experimental and analytical investigations on FRP debonding from concrete substrate have been carried out. A new set-up has been developed for the bond test. With this set-up, bond test can be performed in two different modes: Mode II which is the usual bond test with the interface subjected mainly to shearing and the mixed mode test in which both shearing and peeling are applied to the interface. Note that the latter case simulates the effect of vertical displacement as well as horizontal opening at a shear crack. In the bond tests, strain gauges are attached along the FRP. Based on the variation of strain distribution with increased loading, the interfacial behavior can be deduced. From the mode II test results, it was found that the interfacial shear stress exhibits a sharp drop after reaching a critical value, followed by more gradual reduction with further sliding. Physically, this can be attributed to bond failure followed by frictional decay. In the mixed mode test, an initial inclined angle was introduced to the FRP so applied loading will introduce both shearing and peeling effects. From the test results, two observations were made. The inclined loading angle would cause the FRP to have larger vertical opening but the ultimate debonding load was not affected by the peeling effect. Based on the findings from the Mode II test, a new three-parameter model was developed for interfacial debonding analysis of FRP. In the model, the interfacial shear vs sliding relation is assumed to be governed by three parameters: the maximum shear stress for debonding to initiate, the maximum residual stress right after debonding occurs, and a parameter governing the reduction of shear stress with sliding. After the model was verified through comparison with experimental results, simulations were carried out to study the effect of various parameters on FRP debonding. In real beams, the FRP is subjected to two-way pulling due to multiple cracks formed when the beam is loaded. A two-way debonding model based on the three-parameter model was developed to analyze debonding under multiple cracking situations. The model was used to predict the failure load from a few tests reported in the literature and good agreement has been obtained.