Operational experience has shown how difficult is reaching a reliable Leak Detection System, as several independent studies on real life pipeline incidents have highlighted. Unfortunately, too often several installed systems have disappointed Oil & Gas operators, either because they failed to detect spills on past incidents or because of frequent false alarms. The paper will present an evaluation of different external leak detection technologies (existing and new ones) with respect to the reliability of the detection in real life conditions as well as in challenging environmental or installation conditions. Focus will be placed on potential benefits of new features when adopted as possible means to improve the leak detection reliability, namely : - Digital sensing - Increased sensitivity - Alarm filtering – via adjustment of alarm threshold to avoid nuisance alarms (e.g. due to old leaks or normally present vapors or gases) This paper will also explain the design objectives considered during a particular technology development with goal to achieve a zero-tolerance system: a new generation of elongated digital sensing string. The ability of such a new technology to reliably detect hydrocarbon VOC diffused in the soil ((in addition to liquid sensing) as well as heavy oils even at low temperatures, will be assessed. The dynamic contamination of the sensor element in respect of different hydrocarbons will be presented from a quantitative point of view. The paper will also present the predicted lifetime of the sensing element and the environmental considerations for installations in dry-humid, arctic-tropical conditions, sensor operational temperature range and exposure to water. The practical use of the new sensor will be presented through case studies: - Monitoring of sections of buried crude oil pipeline and reaction to old leaks - Retrofit of facilities for leak detection in double-wall/insulated pipes - Securing the internal leak detection for remote premises