This document is written to serve as a guideline for the testing of microfluidic devices, which are devices designed to manipulate fluids that are confined in a small volume. For instance, a channel with at least one dimension smaller than 1 mm is a microfluidic channel. Practically all designers and manufacturers of microfluidic devices check their devices for leakage. The aim of this document is to provide direction to microfluidic engineers and scientists about when and how to perform leakage testing, a failure mode that is commonly seen in microfluidic devices. In the context of this white paper, leakage is defined as an unintentional outward release of medium to the environment. It is the intention of this group to also create similar documents for internal leakage within or into the microfluidic system.