This report summarizes the results of three years of work on the problems of producing crystals and crystal plus transducer assemblies for long memory time, microwave ultrasonic delay lines. Information on sapphire, spinel, yttrium aluminum garnet, and yttrium boride is presented. At present, magnesium aluminate spinel appears to be the best available material for delay lines. It has low acoustic attenuation and low diffraction losses. Transverse acoustic waves are required in spinel in order to take advantage of its low-loss characteristics. Shear mode transducers of X-cut lithium niobate are suitable for generating these waves. Calculations show that losses in the transducer-to- delay line bond can be held to a minimum by using an alloy of gold, paladium, and silver. This was not verified experimentally. The borides of yttrium are not suitable at present for delay lines because of problems with crystal growth and an anomalous acoustic attenuation that appears to be closely associated with the crystal chemistry.