The significance of Joule-Thomson effect-occurring in throttling-type processes, such as adiabatic flow-in downstream and upstream petroleum industry applications cannot be overemphasized. In this article, the first part of a comprehensive review of this effect in oil and gas field applications is presented. For space reasons, the second part of the review will be presented in a second article. The first part of this review, which is the subject of the present work, includes the Joule-Thomson effect in well testing in oil and gas fields, in applications regarding the horizontal/multilateral/slanted wells, and in drilling, completion, production testing, and monitoring. In addition, challenges of the hydrate formation associated with Joule-Thomson cooling effect are discussed. It is expected that this article would serve as a helpful reference tool for the engineers interested in the Joule-Thomson effect in oil/gas field applications.