Eric Ries is the author of "The Lean Startup" (2011) and creator of the Build-Measure-Learn framework. Ries called principles-of-product-development-flow "quite simply the most advanced product development book you can buy," and Reinertsen reciprocally endorsed "The Lean Startup" as "a brilliant, well-documented, and practical answer" to applying lean principles to startups.
The intellectual relationship runs through several of Reinertsen's core concepts. Ries's emphasis on small batch experiments connects to batch-size-reduction. His Build-Measure-Learn loop is a startup-specific instantiation of fast-feedback. His concept of "validated learning" as the unit of progress echoes Reinertsen's economic-framework-for-prioritization — both argue that the real cost of delay is learning delay, not feature delay. Ries's Lean Startup methodology sits in the lineage from taiichi-ohno → Reinertsen → Ries, translating flow economics from manufacturing to product development to startup business model validation. His original review of principles-of-product-development-flow is preserved in ries-startup-lessons-learned-review, which is a useful primary document of how Reinertsen's ideas were received by the startup community.