Glass fibre-reinforced plastics (GFRP) are composite materials made of a polymer matrix reinforced with glass fibres. The glass fibres therein are treated with a sizing to hold individual filaments together and to promote adhesion to the polymer matrix. Glass fibre-reinforced plastics are covered by Regulation (EU) No 10/2011. Until 31.12.2015, compliance of the sizing was solely based on a risk assessment in accordance with Article 19. This guidance document intends to assist the supply chain actors in determining compliance of sizing formulations with the new requirements of Regulation (EU) No 10/2011.