We warmly welcome you to this truly exceptional conference where the word 'truly' should be emphasized by bold letters. Unfortunately, the worldwide COVID-19 pandemic has stopped the world conference activities for at least one and a half years by closing borders in the spring of 2020, shortly after the EUSFLAT 2019 conference. As a consequence, all conferences moved into fully online schedules. As a result, the genuine conference spirit and atmosphere had disappeared despite enthusiastic and committed organizers' tremendous efforts and dedication. Even as we write these lines, many conferences plan to announce their switching to the online format in the upcoming months.

However, EUSFLAT (European Society for Fuzzy Logic and Technology) and its mother society IFSA (International Fuzzy Systems Association) continue with their commitments and organize an actual conference together with - AGOP (Summer School on Aggregation Operators), IJCRS (International Joint Conference on Rough Sets), and FQAS (International Conference on Flexible Query Answering Systems). In the context of the current situation, our conference looks even stronger than at any time before. Please, recall a long tradition of the meetings - IFSA Congresses have been organized since 1985; EUSFLAT initiated its conferences in 1999 in Palma de Mallorca, Spain; and AGOP, which is the third contributor to these proceedings, began its meetings in 2001 in Oviedo, Spain. Since then, none of these organizations have missed their flagship conferences. What is even more interesting is that all three of them were organized synchronously in the summer of 2019 in Lafayette, USA (IFSA), in Prague, Czech Republic (EUSFLAT), and in Olomouc, Czech Republic (AGOP) – luckily, the world pandemic has not disrupted the pattern. With the utmost efforts and will of organizers, all three organizations have combined their powers and continue their conference traditions by jointly organizing a multi-conference with IJCRS and FQAS in Bratislava, Slovakia, September 19-24, 2021.

None of the conferences has ever been organized in Slovakia, although this magnificent country contributed enormously to the scientific world of fuzzy modeling and aggregation operators. In general, Bratislava and Slovakia deserve special recognition, our respect, and our attention. Therefore, we are thrilled to offer the participants this unique location as the place of this year's IFSA-EUSFLAT 2021 conference and affiliated with it AGOP, IJCRS, and FQAS.

We are extremely glad to present these IFSA-EUSFLAT-AGOP Proceedings with 93 papers. Those 93 papers were carefully reviewed by at least two reviewers each and selected from 124 submissions. Apart from these papers, the conference presents additional talks related to the submitted and accepted abstracts and invited plenary lectures. Altogether, the conference considered 233 submissions across all subevents. This number is indeed extraordinary given the current situation. The fact that the vast majority of participants have chosen the on-site attendance rather than the online one warms our hearts and makes us feel well. We are also encouraged to see so many colleagues from distinct parts of the world, who cannot come to Bratislava, have expressed their interest in the conference and decided to join it in the online form.

Finally, we would like to express our gratitude to all chairs, the organizer team, and especially to Ladislav Šipeky and his agency SIPKES s.r.o. for making it possible that the science in our areas goes on no matter the obstacles.

Radko Mesiar, Marek Reformat, and Martin Štěpnička