In this qualitative study, we investigated how students and teachers responded to the design of a digital self-assessment tool and how they experienced the use of the tool to support understanding and development of twenty-first century skills when working with makerspace activities. There were 65 lower secondary school students and four teachers participating in the study. We used individual interviews with the teachers and group interviews with the students. Data were analysed with thematic coding of transcripts from interviews. The results showed that the participants found that the tool needed technical improvements, but that they gained some insights into the meaning of the included twenty-first century skills (collaboration, creativity, problem-solving, life/social skills and communication). However, it was particularly difficult for students to understand the meaning of life/social skills and how this connected to makerspace activities. Still, both students and teachers argued that it is possible to develop twenty-first century skills during makerspace activities. Teachers also found connections between the skills and learning objectives in the school subject technology, but also in other subjects. We were unsure of how used students were to self-assessment and we believe that teachers’ role is essential, even when it comes to supporting students in self-assessment. Finally, both students and teachers argued that twenty-first century skills are of importance for the future and the project served as an eye-opener in this aspect.