This thesis describes the design and operation of a flexible 'optical parametric oscillator based system, which provides a tunable coherent source of near and middle infrared radiation. The system is suitable for use in applications such as high energy spectroscopy and photo-excitation. The system design has concentrated on obtaining a narrow linewidth and high output energy from the Nd:YAG laser pumped LiNbO3 OPO throughout its tuning range, with the ability to extend operation to longer wavelengths by down-conversion. An important consideration has been to make control and tuning of the system as simple as possible for the experimenter; this has been achieved by the use of motorised tuning mechanisms and programmed digital processor based electronics. Several features, including the nonlinear characteristics of the tuning elements, have been incorporated into the processor software, allowing a simplification of the design of the drive mechanisms and interface electronics to be made. Included are measurements of the performance of the system, including those of the output energy, linewidth, beam divergence and the effects of incorrect tracking between tuning elements, With grating tuning alone, the OPO produces millijoule output pulses of ~3cm-1 linewidth from 1.45µm to 4µm, while the down-converter produces microjoule outputs from 4µm to 24µm. The linewidths of the OPO and down-converter are reduced to ~0.1 cm-1 by the addition of an etalon. The results of some demonstration experiments using the source are also presented, these giving indirect indications of the whole system performance.