What is it to document cross-national longitudinal surveys? In international survey projects, metadata are paramount to evaluate issues of conceptual or methodological equivalence between country datasets. They are crucial also to document subsequent survey waves. This paper presents the ways in which metadata are provided in the framework of the Generations and Gender Programme (GGP). This is a longitudinal demographic survey of 18-79 year olds in Europe and beyond. Survey instruments and guidelines are either adapted to the different national contexts or incorporated into existing surveys. The specific challenge of documenting GGP data is the need to combine information from existing surveys with information on the harmonization process, across different waves. To face this challenge, metadata are provided in compliance with the Data Documentation Initiative (DDI), an international standard for documenting social science data, and are made available in an online codebook and analysis tool through the software package Nesstar. The paper illustrates the DDI elements chosen to document cross-national comparability of GGP surveys and discusses the experience of the GGP team in providing the relevant information. The paper concludes with an outlook on possible developments. In the future, metadata preparation from the beginning of country fieldworks may help optimizing the data documentation procedure.