This paper represents the first of a two-part paper which summarises the research work done on the development of an improved, inverse transient analysis-based, leak detection methodology for water distribution systems (WDS). The work was carried out jointly by Exeter University (EU) and Imperial College (IC) under the EPSRC WITE scheme. In part I (this paper), a summary of the theoretical research work done at EU is presented, while in part 11, a summary of the experimental and further theoretical work done by IC is presented. In this paper, after an introduction and brief background, an inverse transient model (ITM) capable of solving the network calibration problem for leak detection and generally, parameter identification, is presented. The ITM is developed by linking a forward transient model (FTM) with a number of optimisation methods. The FTM developed is based on the method of characteristics solution of the basic transient mass-balance and momentum equations. The ITM capability to detect leaks is demonstrated on a network case study. At the end, a summary is provided with some guidelines for further work.