Animal-based research presents an ethical dilemma between harms (to animals) and benefits (mainly to humans). According to Directive 2010/63/EU, project evaluation should include “a harm-benefit analysis of the project, to assess whether the harm to the animals in terms of suffering, pain and distress is justified by the expected outcome”. By defining benefit and its assessment very generally, legislation opens for a variety of interpretations. In this paper, we review different approaches, consider potential disagreements and highlight the need for a wider discussion of the role of the harm-benefit analysis in ethics review.