Between 1530 and 1660, Ireland and Wales were sites of hope and anxiety for a series of English establishments. While they represented unparalleled opportunities for shining light in 'dark corners', the linguistic, cultural and religious differences of these parts of the Celtic periphery were often perceived to be threats to the metropolitan centre and its various reforming projects. This sense of threat was often felt and sometimes created by Puritans. Puritan movements developed in Ireland and Wales in complex dialogue with their host and sponsoring cultures, sometimes sharing and sometimes opposing prominent themes in English (and, in Ulster, Scottish) Protestantism, and sometimes developing indigenous preferences in piety and theology. In both countries, Puritans faced the challenge of religious apathy and, in the aftermath of the Civil Wars, an enduring royalism. Puritans subjected both countries to a determined push for reformation throughout the 1650s. Nevertheless, while Puritans in Ireland failed to capture the national imagination, their Welsh counterparts succeeded in laying the foundations for later Dissent. This chapter will make a number of general comparisons while looking in detail at the development of Puritan movements in Ireland and Wales, concentrating on the 1640s and 1650s,when Puritan movements became most obvious and when their boundaries became most difficult to sustain.