A Bottleneck is the slowest point in a workflow and determines the pace of the entire system. No matter how fast all other steps work, throughput can never exceed the capacity of the constraint. This is why identifying and resolving bottlenecks is the most effective lever for process improvement.
A simple example illustrates the point. A team has ten developers but only one tester. Items pile up in front of the tester while developers wait for feedback. Hiring more developers would increase the backlog, not reduce it. The right response would be to build testing capacity, whether through a second tester, automated tests, or developers doing testing themselves. On the Kanban board, a bottleneck becomes visible when items accumulate in front of a specific column.
The concept goes back to the Theory of Constraints by Eliyahu Goldratt, which he formulated in his 1984 book The Goal. The central rule is: optimize only the constraint. Improvements elsewhere have no effect as long as the bottleneck remains.
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