Lack of clarity about the proper limits of conscientious refusal to participate in particular healthcare practices has given rise to fears that, in the absence of clear parameters, conscience-based exemptions may become increasingly widespread, leading to intolerable burdens on health professionals, patients, and institutions. Here, we identify three factors which clarify the proper scope of conscience-based exemptions: the liminal zone of 'proper medical treatment' as their territorial extent; some criteria for genuine conscientiousness; and the fact that the exercise of a valid conscience-based exemption carries certain duties with it. These restricting factors should reassure those who worry that recognising rights of conscience at all inevitably risks rampant subjectivity and self-interest on the part of professionals. At the same time, they delineate a robust conscience zone: where a claim of conscience relates to treatment with liminal status and satisfies the criteria for conscientious character, as well as the conditions for conscientious performance, it deserves muscular legal protection.