A major difficulty faced by users of climate data is how to judge whether the data are fit for purpose. This is a serious barrier to widening the use of climate data by non-expert users. Different users require different information, such as reports on validation campaigns, the robustness of the algorithms used, and the data policy. We term this information 'Commentary' metadata. Much work has been done on producing aspects of Commentary metadata, but there is as yet no robust and consistent mechanism to link it to the datasets themselves. CHARMe (“Characterization of metadata to allow high-quality climate applications and services”) will provide these essential links.CHARMe will create a repository of Commentary metadata (hosted by CHARMe or elsewhere) plus a set of interfaces through which users can interrogate the information over the Internet. The project will build some example applications that show the value of exploiting this information in real scientific problems. These functions will be performed through a web portal.The significant and lasting legacy of the project will be: robust and reusable frameworks for linking datasets with Commentary metadata, wherever it is held; reusable software tools that allow climate scientists and users to exploit this information in their own applications; development of best-practice procedures for owners of data archives to exploit these innovations to maximum effect; improved search, intercomparison and time-series analysis tools for large and diverse datasets.The project consortium encompasses data providers, scientists, and developers of future climate services, who participate in major European investments such as GMES, ERA-Clim, ESA's Climate Change Initiative, the Climate Satellite Applications Facility and EURO4M. This partnership is uniquely qualified to ensure that the CHARM system is suited to the needs of diverse user groups, and that existing investments are levered to maximum effect.