This work describes a thirty-two channel programmable transmitter unit for driving an ultrasonic transmitting array using transducers with thickness resonances of up to 2 MHz. It has been developed to allow the performance of time delay focussed transmitting arrays to be thoroughly investigated before their eventual use in an imaging system. The unit will produce a pulse or continuous wave output which is programmable using either a microprocessor or a computer, in both amplitude and delay or phase. The unit's operation is discussed in some detail and the experimental underwater 1 MHz transducer array used for the functional tests is described. Results are presented showing the performance of the transmitting unit when used with this array and demonstrate that the system provides an effective tool by which a proper assessment of time delay focussing may be made. A computer prediction technique·has been introduced. The computer prediction of the field in the region in front of a focussed ultrasonic array has been obtained by the summation of the fields due to the individual array elements. The shape of the short duration acoustic pulses due to the individual elements is determined.by the electrical drive and transducer characteristics. The prediction technique is valid for any pulse shape which can be represented mathematically, however the results presented here have been limited to the pulse shape used in the experimental work. The algorithm used is suitable for a wide range of array formations and the close agreement of the practical and the simulation work shows the validity of the prediction technique. The experimental array has been used as the basis of a detailed investigation into the resolving power of focussed arrays and a number of results have been derived from this investigation. These are used to support extensive simulation studies and computer prediction techniques. The validity of the simulation techniques is assessed and the effect of considering each transducer in the array as a single small but finite sized centrally placed element is compared with that where each transducer is represented by a number of synchronously driven Huygens radiators. The significance of these results with respect to imaging is discussed. The effect on performance of system error is investigated and an assessment of the tolerance of the time delay technique to these errors is made.