Accurate prediction of transitional flow in straight pipes is one of the most challenging CFD applications. While it is not advisable to design fluid equipment to operate in transitional flow regime because of the uncertain flow behaviour, it is recognised that CFD predictions are required in many industrial flow applications where transition controls drag or heat transfer. In this Technical Note, the ESDU CFD validation studies are presented for the prediction of transitional flow in straight, circular pipes with smooth walls, swirl-free, uniform inlet velocity, steady-state fully-developed exit flow conditions, incompressible flow of single-phase, Newtonian liquids and gases, for use of commercial CFD packages typically used in the industry, i.e. finite volume RANS code with low-Reynolds-number turbulence and transition correlation-based 'by-pass' modelling capabilities. A description of the fundamental fluid mechanics of transitional flow in straight smooth pipes is provided and correlations and measurements from the literature are reviewed. The ESDU CFD results are discussed and compared with experimental data and correlations in the literature. Guidance on the CFD modelling of transitional flow based on ESDU CFD validation studies is provided in the Best Practice Guidelines.