This paper charts the development and identifies the core features of a unitary conception of public law that emerged in English law in the wake of procedural reforms in the late 1970s that established the judicial review procedure. This public interest conception holds that the principal concerns of public law are regulation of public power in the public interest and according to precepts of good administration. Private law was distinguished on the basis that its concern was enforcement of private rights and protection of individual interests. This public interest conception underpinned major legal developments in a critical ten- to fifteen-year period following the procedural reform which, together, forged a distinctive, integrated “system” of public law in English law. These developments were radical in a jurisdiction characterised by the Diceyan tradition that public law is private law. On the other hand, it was consonant with the historical development of English law that a substantive legal change was secreted in the interstices of a procedural innovation.The public interest conception has proven hugely influential. It has been used as a normative idea to guide legal development across public law fields. However, the paper argues that this is problematic. This conception was forged by reference to only one branch of public law, the common law of judicial review. Contemporary public law is composed of a range of different bodies of doctrine; it has no functional unity. Where the public interest conception is used to guide development in fields other than common law review, which continues to bear its hallmarks, the risk is that the distinctively valuable functions of those other fields are impeded, while the coherence of those bodies of doctrine may be distorted. The paper analyses two examples of where this risk has become reality: the approach to grant of relief in review proceedings on EU grounds (specifically in the context of EU environmental Directives), and the approach to damages under the Human Rights Act 1998.Ironically, principal lessons to be drawn from the emergence of the unitary public interest conception of public law are that there is no such thing as a unitary system of public law in contemporary English law, and resting legal development on the idea of a general divide between private and public law is likely to lead to fuzzy thinking, incoherence and the warping of legal doctrine; such over-simplifications, while they may appear “neat” or seem “intuitive”, should be repudiated.