This document specifies a system for classifying imperfections that may be encountered in thermoplastic welded joints during manufacture and provides examples of imperfections for the following welding processes: -heated tool butt welding; -heated tool socket welding; -electrofusion socket welding; -hot gas welding; -extrusion welding; -solvent socket welding. This document does not describe imperfections that may be generated during service or imperfections present before welding@ which are due to poor preparation or assembly of components (e.g. fit up). The correct preparation and assembly of components (e.g. fit up) are described in the relevant welding procedure specification (WPS). This document is also not concerned with the search for the possible influence of these imperfections on the behaviour of joints in relation to the different types of stress to which the latter may be subjected or on methods for preventing such imperfections. This document can be used in conjunction with EN 16296 [1] to determine the acceptance of welds. Only imperfections giving rise to discontinuities of materials or changes in shape are taken into consideration in this document@ specifying their type@ their shape and their positions. This classification can be used to determine the possible origin or causes of the imperfections.