This research originated from a mixed methods approach to document the experiences, insights, and perceptions of system leaders involved in a national multi-school system-wide professional learning project, entitled, ‘Leading Collaborative Learning’. This research addresses a central research problem identified by exploratory research questions and further refined in response to a review of the related and relevant literatures. In responding to this research problem, this thesis proposes a distinctive implementation model that incorporates its own impact indicators. There are two main summative findings in this research:System conditions: Five distinctive system conditions are identified as being necessary to sustain the momentum and pace of system-wide school improvement efforts, particularly when the system is faced with the unpredictable, turbulent, and unknown events of the modern world. Implementation model: Two distinctive forms of expertise are identified from the data that bridge the implementation ‘gap’ between policy and practice. The implications for these findings are of interest to policy makers, researchers, and practitioners. Suggestions are made as to how these areas should respond to the findings both now and in the future. This thesis argues that for the Welsh SLO model to be a catalyst for school and system change, it needs to be realised at scale. For this to happen, a deliberate and intentional implementation model must be designed, developed, enacted and, importantly, incorporate its own success indicators. This thesis concludes with a proposed implementation model for the Welsh SLO model that emerged from this research.