What is Complexity?
Complexity is the project characteristic arising from many interacting elements — human behavior, system interconnections, ambiguity — where cause and effect stop being predictable. PMBOK 7 treats navigating it as a core principle: complex is not the same as complicated. Complicated yields to expertise and planning; complex demands probing, sensing, and adapting.
The practical response ladder: decompose what you can, prototype what you can't predict, shorten feedback loops, and stop pretending a bigger Gantt chart will tame emergent behavior.
Worked example
Replacing a data center is complicated: hard, but experts can plan it end to end. Merging two companies' engineering cultures is complex: no expert can predict which teams will gel, so the integration lead runs three-month pilot pairings, watches what emerges, and scales what works. Different beast, different management — treating the second like the first is how mergers write off billions.