One Size Does Not Fit All discusses how project management approaches need to be flexible based on the context and domain of the project. It argues that most projects exist in complex domains that require experimentation and adapting to changing conditions rather than rigid plans. The document suggests that project managers should set expectations based on the reality of the domain, use different steering mechanisms depending on whether a project is simple, complicated, or complex, and transition leadership styles to match the current domain of the project in order to make progress.