A crucial limitation of research on strategic planning is that it has always viewed strategic planning as a single process in a corporation. In practice, strategic planning in complex multi-business corporations has evolved into a network of multi-level and multi-unit strategic planning processes. This makes it challenging for managers and strategists to undertake the activities needed to run those strategic planning systems effectively. The interactions between strategy practitioners as they enact those planning processes play a crucial role in determining effectiveness of the planning process as a whole. Therefore, this thesis is based on a conceptual framework that represents strategic planning as a network of collaboration amongst quasi-independent processes taking place across multiple levels and units. This thesis adopts an embedded design within two in-depth case studies and one pilot case study to examine the strategising activities, practices and interaction dynamics of strategic planning within the M-form based firms. The result articulates the dynamics of strategy practitioners’ interactions in a series of four generic interaction patterns: (1) the Bilateral Scheme, (2) the Cohesive Facilitation Scheme, (3) the Ambassadorial Coordination Scheme, and (4) the Supervisory Driven Scheme. The result also sheds light on the extended roles of strategic planning within a multi-level and multi-unit environment, and on how different actors contribute to the vertical and horizontal aspects of strategic planning. The findings of this research have implications for both theory and practice. This thesis mainly contributes to strategy as practice perspective, strategic planning literature, organisational theory, situated learning literature, sensemaking perspective on practice, power theory, and agency theory. Theoretically, this study introduces a new method for examining the practice of strategic planning based on studying strategic planning links between practitioners representing horizontally as well as vertically differentiated units. In doing so, I have represented strategic planning as a multi-unit as well as a multi-level process, and hence have been able to show how it operates as a network of collaborative relationships and activities. This extends the view of strategic planning prevailing in the literature, which portrays a largely hierarchical, vertically-based structure. Practically, the results provide managers and practitioners with an illustration of how different practitioner roles and managerial levels contribute in distinctive ways to strategic planning from both horizontal and vertical perspectives. It is apparent from my investigation of the case study firms that their planning and decentralised decision-making mechanisms are linked together heterarchically as well as hierarchically.