The present contribution advances the “single top-up tax principle” as a guiding principle for the design of qualified domestic minimum top-up taxes (QDMTTs) by the Inclusive Framework (IF) and by host jurisdictions. This article, which is intended as a critical assessment of the agreed administrative guidance on the QDMTT, generally addresses the justification, content and functions of the single top-up tax principle. It considers both the design of rules on the QDMTT by the IF and implementation issues for host countries, within the permissible range of features for a QDMTT.