This paper reviews some recent work on the detection and sizing of closed internal fatigue cracks by ultrasonic techniques. Major emphasis is put on the diffraction of shear waves at the crack tip. Both fully open as well as partially closed cracks were considered. The effect of crack closure stress on back- scattered (pulse-echo) shear waves was studied with the aid of an A1 compact tension specimen. Noticeable changes with crack closure stress were documented for the structure of both the time- domain and frequency-domain representations. The techniques acquired with this specimen were applied to the study of a 50 μm radius semi-circular crack internal to a diffusion bonded Ti-alloy plate. Improved signal processing techniques were employed to detect the crack and to distinguish it from an artificial surface crack. The probability of detection, assumed to be proportional to the signal-to-noise ratio, was measured as a function of crack interrogation angle and crack closure stress to provide data on optimum probability for detection and sizing. Vigorous research efforts on good models for closed cracks in specific materials and environments are needed to refine the techniques of detection probability.