The present document studies the charging architecture, principles and functionalities for a 5G System Phase 1 specified in TS 23.501 and TS 23.502, based on the TR 23.799 conclusions. The stage 1 requirements from TS 22.115 and TS 22.261 are also input for this study. The charging architecture and functionalities are studied for the set of features and capabilities supported by the 5G System Phase 1, and for interworking scenarios between the 5G System and existing systems as well. Evolution of charging mechanisms and charging models to support the Third Parties are investigated. The study provides a prioritisation between the different features, if evaluated as necessary, to allow a potential phased approach for the normative work.