A multi-regime fluid flow model for internal flows has been applied to several pipe and duct problems. The investigated flow regimes and inter-regime transformations include fully laminar and fully turbulent, laminarization, and turbulentization. The model auto-selects both the flow regimes and the inter-regime transformations. This book studies both steady and unsteady flows, as well as flows in pipes and ducts of both axially unchanging cross section and axially enlarging cross section.