Rigorous peer-reviews are the basis of high-quality academic publishing. Thanks to the great efforts of our reviewers, Machine Learning and Knowledge Extraction was able to maintain its standards for the high quality of its published papers. Thanks to the contribution of our reviewers, in 2021, the median time to first decision was 19 days and the median time to publication was 42 days. The editors would like to extend their gratitude and recognition to the following reviewers for their precious time and dedication, regardless of whether the papers they reviewed were finally published:
| Abubakar, Aliyu | Mahmoud, Karar |
| Ambrosino, Fabrizio | Marchand, Cédric |
| Arteta, Alberto | Marcińczuk, Michał |
| Artiemjew, Piotr | Martínez-Otzeta, José María |
| Bigand, Andre | Matetic, Maja |
| Borza, Diana | Matrenin, Pavel |
| Bozkurt, Aras | Mezzini, Mauro |
| Brandusoiu, Ionut | Milicevic, Mario |
| Byeon, Haewon | Mohammed, Bashir |
| Cabada, Joaquin Gayoso | Moraru, Luminita |
| Carrington, André M. | Moya-Albor, Ernesto |
| Castillo Olea, Cristian | Mrówczyńska, Maria |
| Chalmeta, Ricardo | Muncharaz, Javier Oliver |
| Chen, Liang-Bi | Naqvi, Rizwan Ali |
| Choi, Minseok | Nayak, Sridhara |
| Conflitti, Paolo | Nayel, Hamada A. |
| Crisan, Gloria Cerasela | Ojeda, Dora Luz |
| Damaševičius, Robertas | Panek, Jarosław |
| Delnevo, Giovanni | Pejic Bach, Mirjana |
| Demertzis, Konstantinos | Piga, Dario |
| Deriu, Marco Agostino | Podda, Marco |
| Derlatka, Marcin | Pouliakis, Abraham |
| Dowling, Benjamin | Radac, Mircea-Bogdan |
| Egger, Jan | Rana, Pratip |
| Fakotakis, Nikos | Riguzzi, Fabrizio |
| Feng, Chaochao | Rossi, Riccardo |
| Garcke, Jochen | Saeed, Khalid |
| Gomes, Rahul | Samakovitis, Georgios |
| Gornicki, Krzysztof | Schreiber, Andreas |
| Gosti, Giorgio | Sciuto, Grazia Lo |
| Howe, Bill | Seeland, Marco |
| Hsin, Kun-Yi | Shan, Hongming |
| Jeong, Young-Seob | Shi, Feng Bill |
| Jurman, Giuseppe | Temerinac-Ott, Maja |
| Kanavos, Andreas | Tran, Minh-Quang |
| Kapociute-Dzikiene, Jurgita | Trujillo, Logan T. |
| Kertész, Gábor | Tsaramirsis, George |
| Kieseberg, Peter | Turkay, Cagatay |
| Kiourt, Chairi | Valero-Mas, Jose J. |
| Klimaszewski, Krzysztof | Veloso, Bruno Miguel |
| Klimova, Alexandra | Wang, Hsiang-Chen |
| Le, Nguyen-Thinh | Wang, Kai |
| Leo, Marco | Wood, David |
| Li, Guanpeng | Xu, Hongming |
| Lin, Guo-Shiang | Yazdani, Maziar |
| Lisjak, Dragutin | Yoo, Jung Sun |
| López-Yáñez, Itzamá | Zaborowicz, Maciej |
| Luk, Robert | Zambrano-Martinez, Jorge Luis |
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