Accounting for nationally determined contributions (NDCs) is an important component of the ‘rule-book’ required for effective implementation of the Paris Agreement. Robust mitigation accounting is needed for effective implementation and to strengthen the multilateral rules-based regime under the Convention. This paper focuses on mitigation accounting as a particular component of the Paris Agreement Work Programme (PAWP), while also identifying linkages to other elements such as transparency. Some working definitions of key terms are used in this paper – what we mean by counting, accounting and accounting for NDCs; and when accounting is applied. In this paper, counting refers to quantifying data, parameters and assumptions, and performing mathematical operations; accounting generally means comparing against a goal or benchmark; and accounting for NDCs means comparing achievement of the targets in a Party’s NDC and explaining this in a responsible manner. The timing of application may be understood before, during and after a period of implementation (that latter term used interchangeably with common time-frame). Before a period of implementation, a Party should describe the accounting methodologies to be used, as part of the information communicated with its NDC (negotiated under APA item 3 b). During a period of implementation, a Party will provide information that enables tracking progress on implementing its NDC. After a period of implementation, a Party will report information on achievement of its NDC.