In this paper, I will examine the extent to which the common law of tort in England and Wales imposes a duty to prevent harm on public authorities and private individuals. As will be seen, the starting point for the common law is that such liability should, in both cases, be regarded as exceptional. This must, however, be weighed against duties to prevent harm that arise under the torts of negligence and breach of statutory duty. Public authorities may also face claims that their failure to prevent harm is in breach of ECHR arts 2 or 3. While the law is complex, this paper identifies three key arguments that explain the current legal position at common law, namely that: (i) tort law should treat private and public parties alike: (ii) human rights claims should be treated as distinct from private law claims and (iii) libertarian concerns signify that a duty to prevent harm should be exceptional and needs to be justified. While these arguments provide both an explanation of and a justification for the current law, this article questions to what extent the treatment of public authority liability may be regarded as unduly harsh on vulnerable claimants.