This thesis considers aspects of contextualisation in the mission of local churches in twenty-first century Wales. Welsh Nonconformity rose rapidly to a dominant position in Welsh society and culture in the nineteenth century, but has subsequently declined equally rapidly. By the beginning of the twenty-first century its total demise is predicted. The research examines the contextual factors in this decline, and their relevance for possible recovery. Contextualisation is an essential part of missiology, in calibrating appropriate mission to the distinctives of a particular nation or locality. Wales is shown to be a distinctive context for mission, both nationally and regionally, in relation to specific aspects: religious, geographic, ethnic, linguistic, cultural, social and political. Contextual studies have been done for other mission contexts, but not for Wales. This research seeks to address this lack. The thesis first outlines the development of the main approaches in global mission, their underlying assumptions, and their outworking in the mission of local churches in the West. The approaches have been identified as Evangelistic, Lausanne, Missio Dei, Liberal and Emergent. Drawing on hundreds of questionnaire responses and extensive interviews with Nonconformist leaders, the research examines how the different approaches to mission have been expressed in Wales, and how each approach adjusted to each aspect of context. The growth trends of the different approaches, patterns of church and mission, and adjustments to Welsh contexts in the first decade of the twenty-first century, or not, are then examined. The resulting analysis enables good practice to be identified, and approaches for effective mission suggested for the coming decades.