The application of the theory of voltage regulation on a distribution network to the problem of optimising the various voltage control facilities available is generally assumed to be well understood. However, with complex network arrangements, the wide variety and numbers of distribution transformers, and differing load patterns, a distribution engineer is faced with a major problem when attempting to optimise the voltage performance of a distribution network, while maintaining the voltages supplied to consumers within declared limits. The paper covers a theoretical approach to distribution network voltage performance, leading to the derivation of a parallelogram theory linking primary substation source busbar voltage and the tap setting on each distribution transformer on the network. Based on this parallelogram theory, a computer program has been developed which enables distribution networks to be surveyed in greater depth, thus enabling engineers to more readily identify problem areas. Overall the program provides a powerful and efficient means of assessing and optimising the voltages on existing networks, or modifying networks to accommodate additional loads, or for designing new networks. The program is adaptable for use on any MV/LV distribution network. The paper thus provides both a theoretical and practical approach to the problem of improving the voltage performance of distribution networks.