"Over the years, numerous ethical implications in stem cell research have been identified. Consequentially, there is a need to anticipate, prevent and/or mitigate these implications. In literature and in the guidelines of the International Society for Stem Cell Research some of these implications have been reframed into (moral) responsibilities. What exactly is meant by responsibility and which notions of responsibility are important often remains unclear. As a consequence, this encumbers the possibility to deal with these responsibilities in a systematic way. Therefore, the goal of this paper is to discuss how the concept of responsibility in stem cell research could best be understood. This paper addresses which notions of responsibility are relevant for the field of stem cell research. This will be done by first distinguishing between backward-looking and forward-looking notions of responsibilities, where backward-looking responsibilities are about reactive attitudes and forward-looking responsibilities are about what could be expected from someone to prevent ethical implications. Subsequently, ethical implications of and wrongdoings in stem cell research will be categorized in these notions of responsibility. Taking one step back and looking at the different notions of responsibility could help to identify gaps in responsibilities as well as to distinguish obligatory and supererogatory responsibilities. Consequentially, this informs how to prioritize, distribute and delegate responsibilities over the different stakeholders in stem cell research. This paper concludes by discussing the distribution of responsibilities and different strategies to promote responsibility in stem cell research. "