This reflection paper is intended to reflect the current experience of the Quality Working Party (QWP), of the Committee for Medicinal Products for Human Use (CHMP) and the Co-ordination Group for Mutual Recognition and Decentralised Procedures-Human (CMDh) concerning the definition of a New Active Substance (NAS) in the context of preparation of dossiers and submissions of applications for Marketing Authorisation (MAA) in the Centralised Procedure (CP), the Mutual Recognition Procedure (MRP)/Decentralised Procedure (DCP) and purely national procedures for chemical medicinal products for human use.