This thesis uses cultural historical activity theory (CHAT) and a combination of ecological, relational and authorial perspectives on agency to explore the practice of project-based STEM activities in English secondary schools. While the educational consensus in England has shifted back in favour of traditional teaching methods and away from “progressive”, student-centred pedagogies including PBL, STEM activities are still considered by many to be a viable strategy for promoting pupil engagement and interest in STEM subjects. This study explores what place STEM PBL activities can have in schools, what needs and motives they are perceived to meet, and how they are implemented in complex school environments. Cultural historical activity theory prompts us to consider STEM PBL activities as historically emergent, object-oriented social practices, situated among networks of other activities and shared objectives. Meanwhile the ecological perspective offers a challenge to the linear consumerist ‘pipeline’ metaphor that is common in STEM policy discourse. This thesis positions teachers as critical agents within their local STEM ecosystem and pupils as purposeful agents who author their own interest pathways and experiment with ways of acting in the world through STEM. The research was conducted as a series of three case studies incorporating a range of data collection methods. A hybrid analytical approach combining reflexive thematic analysis (Braun and Clarke, 2006) and Engeström’s (1987) third generation activity theory framework was applied to provide both a system level view and a broad, multi-voiced account of the cultural context and participant perspectives. The findings indicated that teachers face a contradiction between meeting the demands of curriculum and qualification targets, and teaching in ways that they enjoy and believe to be meaningful for pupils. STEM projects and challenges delivered in enrichment time (lunchtimes or after school) were seen to offer an antidote, an opportunity to break away from the status quo, to explore new practices, develop new communities and relationships and to foster pupil agency in a low risk environment. The relative difficulty of establishing successful activities was found to be strongly influenced by the maturity and diversity of the school’s wider STEM ecosystem and the number of ‘keystone’ teachers involved in planning and sustaining engagement across the school. Teachers were found to exercise considerable agency in drawing upon their past experiences, present resources and future aims, as well as making strategic use of externally provided structures and incentives. As a result, pupils were afforded opportunities to co-author their own activities to various extents by acting through their environments and agentively navigating and negotiating the resources available to them. Bringing together the insights from a survey of the policy context in England, a review of the literature on STEM PBL, and the theoretical and empirical work in this study, it is argued that spaces for teachers and pupils to engage with STEM in creative, collaborative and agentive ways are more important than ever. I suggest that STEM PBL activities are not a challenge to the importance of a knowledge-rich curriculum, but a valuable addition to it, giving life and meaning to the tools and expertise of STEM disciplines while allowing pupils to construct their own interest pathways and ‘try on’ new roles and identities. It is further argued that the combination of STEM ecosystem thinking and the activity theoretical framework provides tools for understanding how pupil agency can be facilitated and how PBL can be developed and embedded in specific local contexts.Economic and Social Research Council (ESRC)