This paper presents an analytical model for investigating the effect of debonding between piezoelectric actuators/sensors and the host beam on the sensing and actuating behaviour. In this model, both flexural and longitudinal displacements of the host beam and the piezoelectric layers are considered using classical beam theory. For the adhesive layer, only through-thickness peel strain and transverse shear strain are taken into account, and they are assumed to be constant across the adhesive thickness. When a debonding occurs between the host beam and an actuator or sensor, the stresses transferred via the adhesive layer are assumed to be zero. Using this model, the effects of debonding on the sensing and actuating behaviour are examined. Numerical results reveal that debonding can have remarkable effects on the distributions of strains and internal forces as well as frequency spectrum and charge output.