Transmission error is a measure of the positioning accuracy of a gear system. This has been widely documented in gearing for many years as the source of problems in noise and vibration. It is a result of errors in the contact conditions which affect the driven gear with respect to the rotation of the driver gear. This research aims to present a better understanding of the basic kinematics of worm gear systems by identifying the significant influences which determine the contact conditions. A literature review of existing theory is described which determines the major areas considered in worm gear contact analysis. Formulae are derived which quantify the effect of component parameter variation on contact. An investigation of the design, manufacture, and operating processes is recorded which identifies error sources relative to the theoretical contact condition. A computer program is developed which calculates contact characteristics such as worm and wheel component form, transmission error and contact marking pattern for a given design including any contact error sources. Computer calculations are validated by comparing direct measurements of these characteristics from several manufactured gear sets with synthesised results produced using the design information, machine settings and error sources detected during production. The behaviour of these gear sets during operation under a torque load has been investigated experimentally. Measured transmission error data from a test rig is used to develop a basic model of worm gear deformation under load. This model has been added to the computer program to improve and extend the analysis capability. The test rig has also been used to investigate the effect of initial wear on contact characteristics. The good correlation between calculated and experimental results shows that the characteristics of a worm gear set can be predicted once all elements of the design and manufacture are known. The results also validate the software as a useful design tool for academic and industrial applications. Important conclusions are drawn on design techniques, the manufacturing process, and the effects of operating under load. Further areas of investigation are identified which offer future research an opportunity to expand upon these conclusions.