This article examines the series of legislative measures, beginning in 1776, which culminated in the passage of the Penitentiary Act of 1779. It argues that, although the Penitentiary Act is of considerable long-term significance in the history of English criminal justice and penal practices, the act passed in 1779 was in fact a somewhat modest affair by comparison with the scheme originally envisioned by its principal architects. The act embodied a decisive retreat from an original ambition to replace transportation with imprisonment at hard labour as the principal punishment next to death in late eighteenth-century England. This modification arose from a pragmatic appreciation of the limitations imposed, first, by a persistent preference amongst most legislators for transportation of the worst classes of offenders not actually put to death and, secondly, by the reluctance of local authorities to have such a preference imposed upon them to the detriment of local control of punishment and of the finances which paid for it. Attention is also drawn to how the course of events was shaped by the interaction of the act's main architects, William Eden and Sir William Blackstone, with both government and non-ministerial MPs such as Sir Charles Bunbury.