We present details of the technical design, manufacture and testing of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a twophase
xenon detector which measures both the scintillation light and the ionisation charge generated in the liquid by interacting particles
and radiation. The instrument design is driven by both the physics requirements and by the technology requirements surrounding the use
of liquid xenon. These include considerations of key performance parameters, such as the efficiency of scintillation light collection, restrictions
placed on the use of materials to control the inherent radioactivity levels, attainment of high vacuum levels and chemical contamination
control. The successful solution has involved a number of novel design and manufacturing features which will be of specific use to
future generations of direct dark matter search experiments as they struggle with similar and progressively more demanding requirements.