This paper considers the role played by the Commons Preservation Society (CPS) in defending Berkhamsted Common in Hertfordshire from enclosure 1866–1870. The CPS was an organised pressure group, which deployed a series of legal and historical arguments to defend open spaces from development and to encourage anti-enclosure legislation. At Berkhamsted the CPS orchestrated a fence break in March 1866 to oppose forcibly the enclosure of a portion of the common by the trustees of the second Earl Brownlow. The primary purpose of the paper is to demonstrate how material gathered by lawyers working for the CPS in the course of the subsequent legal dispute over the fence break can be used to throw new light upon the uses of common lands in the later nineteenth century. A secondary aim of the paper is to consider the longer-term impact of the CPS on debates about common land and enclosure, and to point to the need for further research into the origins, motivations and activities of the Society.