The object of the work is a combined theroretical and experimental study of diffraction of Lamb waves by two- and three-dimensional cracks in a plate. An important motivation for this project is the desire to develop a technique capable of detecting and characterizing cracks in weldments in plates. We want to analyze the dependence of the scattered signals on the dimension, angle of inclination and depth of burial of the crack. The basic measurement techniques requires the accurate determination of minute normal surface displacements at ultrasonic frequencies. Such measurements have recently been made possible by the development in our laboratory of a new surface displacement capacitive tranducer [1]. The measured displacements will be compared with the theoretical predictions.