In this paper, an in situ technique for the localisation and the sizing of cracks in plate-like structures is presented. The excitation and the reception of the ultrasonic diagnostic signal – Lamb waves – are achieved by permanently installed piezoelectric ceramic disks. The technique simplifies the underdetermined problem of the full characterisation of cracks to an over-determined problem of the localisation of crack tips. Also, it overcomes the difficulties associated with using the pitch-catch excitation scheme for crack characterisation. The development of the technique was done systematically using analytic models and the finite element method. The technique has been validated by an experiment and assessed by extensive parametric studies. While it was designed to be a convenient approach, highly accurate crack characterisation has been attained as evident through the results of both numerical studies and physical experiments.