Theoretical considerations are presented for the determination of disbonds at an adherend/adhesive interface using ultrasonic techniques. Two simplified models are presented to relate the adherend/adhesive bond stiffness to the total disbond length at the adherend/adhesive interfaces. In addition, the analysis of Goland and Reissner for an adhesive lap joint has been extended to the case of a distribution of through-the-thickness disbonds in the adhesive layer. The shifts in the frequency minima of the reflection coefficient versus the total disbond length have been calculated. For the model in which the disbonds are located at arbitrary positions at the two adherend/adhesive interfaces, the theoretical results are shown to agree with the experimental results.