This exploratory thesis aims to describe and analyze how boundaries emerge, affect planning practices, and how boundaries are managed over time in order to achieve integrated planning processes. This thesis is particularly interested in how the interplay between boundary-spanning and boundary-crossing activities in relation to the organization of strategic spatial planning can be understood from an institutional perspective. The overall research question was sub-divided into three further research questions: How do boundaries emerge, and how are they managed, when planning is organized? What boundary arrangements emerge over time for the managing of boundaries? How can the choice of boundary arrangement be understood in relation to institutional orders? The thesis involves a case study of two urban planning projects and an organizational planning reform. Using a qualitative research approach, empirical data were collected through participant observations when planners collaborated, shadowing planners in their work environment, conducting semi-structured interviews with planners, hosting focus group discussions, and analyzing relevant documents. The findings of the present study demonstrate that planners face multiple challenges related to boundaries, which in turn encourages continuous transformation processes. Negotiations are revealed to play a crucial role in managing and modifying boundaries. More specifically, different and conflicting views on how boundaries should be drawn and desired characteristics of the boundaries converge in negotiation processes. Drawing on the institutional theoretical framework of the study, the organizing principles of egalitarianism and hierarchy are positioned in opposition to one another. Negotiations ultimately result in an egalitarian approach to boundary management, which involves preserving formal boundaries and organizing planning work across boundaries. An informal planning organization emerges alongside the formal planning organization, and specific boundary arrangements and approaches to organize collective action in a polycentric planning environment become institutionalized. This development, however, gives rise to new problematic boundaries between the formal and informal planning organization, which in turn leads to further negotiation and incremental changes to boundary arrangements and planning practices.