Factors influencing the transition of knowledge-intensive SME start- ups scale-up have become a major area of interest to academics in entrepreneurship, policy-makers and practitioners. Yet we know little about the specific HR actions, behaviours and capabilities needed to surmount the management challenges faced by scale-up firms. Thus we ask: how and when can HR actions, behaviours and capabilities transform start-ups into scale-ups? Our findings are based on longitudinal, ‘engaged’research with a high-tech scale-up. Drawing on this research, we develop a process theory that provides insights into how start-ups can transform organically into scale-ups by valuing people, creating sustainable working conditions and co- evolving with their entrepreneurial ecosystems. We elaborate on the presence of three stages in this process that are potentially generalizable to other SMEs scaling-up: laying the human capital foundations; achieving scale through bridging and bonding social capital; and re-defining the entrepreneurial eco-system through enhanced reputational capital. Finally, we discuss the contributions of our research to theory and practice in entrepreneurship and HRM."