Nuclear Decommissioning Projects (NDP) are characterised by an intrinsic complexity that leads to specific challenges, thereby amplifying the overall difficulty of such undertakings. Unit 5 is specifically dedicated to the topic of Managing Complexity in NDPs: Key Challenges. It begins by offering an overview of NDP processes and the associated generic challenges, with particular emphasis on the need to strike a balance between flexibility and rigorous planning in order to maintain safety and efficiency over time. Secondly, Unit 5 revisits risk analysis in the context of the transition from operational status to complex decommissioning projects. Building on these foundations, the unit then focuses on three particularly prominent challenges within decommissioning projects: waste management strategy, human resource management, and knowledge management.