The work, of which this thesis is a record, is concerned with the development of a microprocessor-based signal analyser for machine condition monitoring. Until recently, the technology did not exist to produce such an instrument in a 'semi-portable' form. The work proceeds with a revision of condition monitoring and digital techniques which may be implemented on such an instrument. These techniques can be used to detect such faults as: bearing wear; out of balance; shaft misalignment; damaged gears and electrically induced vibration. The greater part of this work involves: firstly, establishing a suitable hardware architecture for the instrument and, secondly, implementing the digital signal processing algorithms required. Such a system, capable of implementing both time and spectral techniques, has been developed to laboratory prototype level. The system consists of a high-speed, multi-channel data acquisition unit and both 8-bit and 16-bit microprocessor systems. The microprocessors execute the operating system and signal processing software. The aim, to produce a simple to use and flexible instrument, was sustained throughout the design phase. The result is an instrument which should offer multi-functional protection of plant.