Collaborative planning has proven to be effective in areas where complex problems are linked to multiple stakeholders and entities, like urban planning, military, and supply chain management. In project management, however, the concept has been less present, and few studies have explored its implications for an improved project planning process. This article uses a qualitative multiple case studies approach to explore the dimension and requirements of collaborative planning for business transformation projects. Two dimensions are identified: planning for collaboration and planning in collaboration. A set of requirements are also identified to enable both dimensions of collaborative planning.