In this Technical Note, the ESDU CFD validation studies are presented for the prediction of friction losses and flow characteristics in straight circular pipes with smooth walls. Accurate prediction of the friction losses and flow characteristics in straight pipes is one of the most challenging problems in CFD. In this Technical Note, guidance is provided on the CFD modelling of friction losses and flow characteristics in straight circular pipes with smooth walls, laminar, transitional and turbulent flow regimes, swirl-free, uniform inlet velocity conditions, fully-developed exit flow conditions, steady-state 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 'bypass' modelling capabilities. Correlations and measurements from the literature for friction losses and flow characteristics in straight pipes are reviewed. The ESDU CFD predictions for the different meshes, pipe lengths, Reynolds number and turbulence models are discussed and compared with experimental data and correlations. Best Practice Guidelines for CFD modelling in laminar, transitional and turbulent flows are provided.