Rigorous peer-reviews are the basis of high-quality academic publishing. Due to the great efforts of our reviewers, Challenges 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 28 days, and the median time to publication was 51 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:
| Adam Glowacz | Katarzyna Pawlewicz |
| Agnieszka Trystula | Katherine Arbuthnott |
| Akansha Singh | Kim Dooley |
| Alan C. Logan | Letizia Appolloni |
| Alexandre Szabo | Luke J. Ney |
| Ana Ancheta Arrabal | Mariusz Urbański |
| Bibhuti K. Sar | Marta Brković Dodig |
| Caven M. Mnisi | Martius Rodriguez Y Rodriguez |
| Charles Tijus | Matthew Schneider |
| Dana Badau | Mauro Lizot |
| Diego Boerchi | Mayra Antonelli-Ponti |
| Douglas Hanes | Mehmet Onder |
| Eduardo Jiménez-Fernández | Michael Yellow Bird |
| Geoffrey Crisp | Michał T. Tomczak |
| Giles Sioen | Nilza Costa |
| Helen Cowie | Pedro A. Martin Cervantes |
| Hoi Cheu | Rishika Sakaria |
| Hugh Whiteley | Robert Frodeman |
| Ines Villano | Sara Encarnação |
| Inna P. Solyanikova | Sebastian Zahnreich |
| Izabela Bolesławska | Sheldon Krimsky |
| Jacek Chmielewski | Silviu Nate |
| Jamile Sabatini-Marques | Sina Hafizi |
| Janusz Rak | Solly Benatar |
| Jarosław Chudzicki | Sorin Burlacu |
| Jarosław Pinkas | Stanislav Benčič |
| Jennifer Cole | Tan Yigitcanlar |
| Jens Jelitto | Tomasz Sobierajski |
| Johannes Berens | Valdemar E. Arce Guevara |
| John Penders | Vincenzo Marzullo |
| José Miguel Molina Martínez | Yuta Shibamoto |
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