The concept of boundary work (Gieryn 1983, 1999) has been developed to capture theways in which scientists collectively defend and demarcate their intellectual territories.This article applies the concept of boundary work to the ethical realm and investigates theethical boundary work performed by researchers in the field of citizen science (CS) througha literature review and by analysing accounts of ethics presented in CS literature.Results show that ethical boundary work in the CS literature is, to a large extent, a matterof managing ambiguities and paradoxes without any clear boundaries drawn between theunethical and ethical. Scientists are negotiating ethical positions, which might, occasionally,enhance the ethical authority of ‘non-science’ and non-scientists, as well as maintainalready established research ethics. The main ethical boundary work in CS displaysvariations towards perceived insufficiencies of conventional research ethics to accommodate“outsiders”, addressing issues of distribution, relevance, and expulsion as science includevolunteer contributors in the scientific process.