Management innovations are an important source of competitive advantage, but we lack knowledge on the implementation process, not least in small- and medium-sized companies (SMEs). Recognising that psychological ownership (PO) represents a crucial aspect of the implementation process, we address micro-foundational characteristics of the implementation process. PO and critical incident theory (CIT) provide a lens enabling this micro analysis. The empirical setting is the implementation of Hoshin Kanri, a strategic management system in eight small companies. From the analysis of the eight cases, we operationalise four dimensions that characterise how PO evolves in the implementation process: types of PO incidents, frequency of PO incidents, incidents indicating an increase or decrease in PO, and incidents addressing individual or collective PO. Looking at how PO is developed both among CEOs and managers in SMEs, we use the four dimensions to characterise the evolvement of PO within the focal organisations. In doing so, our article elaborates on PO as a driver and, if insufficiently developed, an impediment to effectively implementing management innovations.