The first Joint World Climate Research Programme (WCRP)–World Weather Research Programme (WWRP) Symposium on Data Assimilation and Reanalysis took place on 13–17 September 2021, and it was organized in conjunction with the ECMWF Annual Seminar on Observations. The last WCRP–WWRP-organized meetings were held separately for data assimilation and reanalysis in 2017 (Buizza et al. 2018; Cardinali et al. 2019). Since then, common challenges and new emerging topics have increased the need to bring these communities together to exchange new ideas and findings. Thus, a symposium involving the aforementioned communities was jointly organized by the German Meteorological Service (DWD), Hans-Ertel-Centre for Weather Research (HErZ), WCRP, WWRP, and the ECMWF Annual Seminar. Major goals were to increase diversity, provide early career scientists with opportunities to present their work and extend their professional network, and bridge gaps between the various communities. The online format allowed more than 500 participants from over 50 countries to meet in a virtual setting, using the Gather platform (www.gather.town) as the central tool to access the meeting. A virtual conference center was created where people could freely move around and talk to other close-by participants. A lobby served as the main hub, and it connected the poster halls and the conference rooms for the oral presentations and the ECMWF seminar talks. The feedback from the participants was overwhelmingly positive. Scientifically, the meeting offered opportunities to bring together the communities of Earth system data assimilation, reanalysis, and observations to identify current challenges, seek opportunities for collaboration, and discuss strategic planning on more integrated systems for the longer term. The contributions totaled 140 oral and over 150 poster presentations covering a large variety of topics with increased interest in Earth system approaches, machine learning, and increased spatial resolutions. Key findings of the symposium and the ECMWF Annual Seminar are summarized in the next section. The “Conclusions and remarks” section highlights the common and emerging challenges of these communities.