The determination of the depth of surface-breaking cracks in concrete specimens using an ultrasound diffusion technique is discussed. Experiments were carried out on pre-cracked concrete specimens of varying crack depths (0 - 40% of the specimen thickness). Contact transducers were placed at the specimen surface with source and receiver separated by the crack. Tone bursts excitations over a frequency range of 400-600 KHz were used. At these frequencies, ultrasound is scattered considerably by the heterogeneities in the concrete. In the limit of many scattering events, the evolution of energy may be modeled as a diffusion process. The arrival of the peak diffuse energy at the receiver is delayed due to the presence of crack. This delay is the prime indicator used for determining crack depth. Several data reduction methods were explored and are discussed. Numerical and analytical analyses were also used for comparison. These results are in basic agreement with the experiments. In addition, these analyses are used to study the limits of this technique. In particular, it is shown that this technique is applicable to cracks greater than the scattering mean free path, which is estimated at 1 cm for these specimens. Aspects of practical implementation are also discussed.