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, conscientious exemptions may become increasingly widespread, leading to intolerable burdens on practitioners, patients, colleagues and institutions. Here, after explaining our understanding of conscience and setting out a defence of the role of conscience based exemptions in healthcare, we identify three restricting factors which clarify the proper scope of such exemptions. First, we identify the territorial extent of conscience-based exemptions as being the margins of proper medical treatment, and argue that such exemptions are appropriate only where treatment has a liminal status. Secondly, we clarify the criteria for conscientiousness and, finally, we consider the conditions under which any valid conscience-based exemption must operate. These clarifications should help to reassure those worried that recognising rights of conscience at all inevitably risks rampant subjectivity and self-interest on the part of professionals. At the same time, the restricting factors we identify also have the effect of delineating a very robust conscience zone. Where a conscience claim relates to treatment of liminal status and satisfies the criteria for conscientious character, as well as the conditions for conscientious performance, it deserves very muscular legal protection indeed.