Apart from regulatory and statutory requirements, the motivation to run a Leak Detection System (LDS) on a pipeline or piping system is continuous monitoring of system integrity and preparedness for fast initiation of countermeasures in case of a detected and confirmed leakage. Fast and effective mitigation measures can reduce adverse impact on the environment and will substantially reduce the cost for restoration. Under normal operating conditions the LDS is permanently confirming the tightness of the pipeline or piping system. In case a leakage occurs, the LDS notifies the operating personnel through messages and alarms about the leak rate and its location, so that fast and effective mitigation measures can be initiated. It is important to realise that a LDS cannot prevent leaks. It is part of the emergency response in case all measures for leak prevention failed. There are various methods for leak detection, each of them with specific strengths and weaknesses. And not all methods suit all pipeline or piping systems equally. Besides conventional so-called Internal Leak Detection (ILD) methods, including Computational Pipeline Monitoring (CPM), there are new promising developments with so-called External Leak Detection (ELD) systems. This article gives a brief overview and comparison between ILD and ELD methods, and how they could complement each other. The selection of the optimum set of leak detection methods for an individual pipeline system is a non-trivial engineering task. This article is intended to serve as a guideline for preliminary considerations on LDS selection. It is intended to support all those who are planning to establish a new LDS or to revamp an existing LDS. Most of the statements in this article can be used for piping systems in general. This article, however, is focusing on long-haul underground pipelines for transportation of oil, gas and other fluids.