This essay has been written by Dr Jonathan Edis. It does not seek to challenge the role of harm as a central component of national policy in making planning decisions affecting the significance of heritage assets. Rather, it seeks to challenge our understanding of harm. It questions whether we have adequate means of distinguishing between harm and change, whether we can calibrate harm well enough, and whether the way in which we balance harm with benefit is sufficiently rigorous. We have been aware of the potential for harm to occur to heritage assets for a relatively long time, but it is only in the last decade or so that we have been called upon to use harm as the guiding light of assessment. Harm contains paradoxes. It is not a fixed measure, and there is no agreement or formal guidance as to the spectrum or scale by which it might be described. The relationship of harm to other significant environmental impacts could be better understood. In these circumstances it is not unreasonable to ask questions about harm and its characteristics.