This document (“Guiding Document V.1” or “paper” hereafter) is a component of the project Identifying and quantifying plastic contaminant sources and leakages into the aquatic environment1, executed by UNEP International Environmental Technology Centre (UNEP-IETC) in cooperation with the European Commission Directorate-General for the Environment (DG ENV) and supported by the European Commission. The main project’s objective is to develop a practical methodology to identify and quantify sources of plastic pollution in the aquatic environment, allowing the production of an inventory of plastic pollution from a plastic waste source perspective, so that decisionmakers and control bodies can better understand current scenarios and develop strategies to address plastic pollution, with a particular interest on aquatic environments and focus on tailored actions according to pollution sources. The priority on sources of plastic pollution in aquatic environments is because there are many models and information available on this regard, including environmental aspects like riverine fluxes of plastic pollutants to coastal environments. This Guiding Document (V.1) has been built upon the Scoping Document that introduced the initial conceptual framework for the toolkit development, aiming to set the model for the establishment of its boundaries and provide a technical basis for the toolkit.