ISO 21248:2019 defines terms for the quality assessment of national libraries and specifies the following methods for the assessment:— performance measurement, and— impact assessment.The results of both methods are of special interest for comparison over time within the same library. Comparisons between libraries are possible if differences in the mandate, tasks and constituencies of the libraries are taken into account.Not all methods described in this document apply to all national libraries. Limitations of the applicability of individual methods are specified in the descriptions.This document is not intended to exclude the use of performance indicators or of methods for impact assessment not specified in it.This document does not cover web archiving, but refers to ISO/TR 14873 for statistics and quality issues for this new task of national libraries.