This paper deals with a case study on how to achieve sustainable e-learning through systematic certification. The case is a nationwide Danish project on the certification of 23 vocational e-learning courses conducted by ‘The National Danish Knowledge Centre of e-learning’ in 2018.The research objective is to investigate the pros and cons of this e-learning certification. How and why does it promote sustainable e-learning and professional development among teachers in vocational education and training?The research results provide evidence suggesting that the certification increases the sustainability of vocational e-learning. In particular, it promotes consistency between expected learning outcomes, preconditions as regards content, duration, flexibility, use of digital platform etc., and design in terms of organization and evaluation of learning activities.The certification of e-learning also fosters professional development among the teachers involved. They appreciate the systematic nature of the certification process and the template applied as well as the provision of supportive and specific feedback from the certifier.When the vocational teachers were assigned the role as e-learning designers, they stepped out of their comfort zone. In these cases, they greatly appreciated receiving supportive and challenging feedback. The combination of support and pressure helped them move forward towards new experiences and broader repertoires of sustainable e-learning methods.In particular, the teachers benefitted from single-loop learning, which improved their planning actions, and double-loop learning, which improved their beliefs about sustainable e-learning. These results can be generalized and expanded to other projects. Consequently, the certification is important for the future development of sustainable e-learning.