Over the last three decades, computational fluid dynamics (CFD) has matured into a reliable tool for analysing fluid-flow problems. However, CFD does not provide an exact solution to the problem because of various approximations involved. Problems arise due to errors and uncertainties involved in CFD simulation. Commercial CFD codes provide many default settings that may not be appropriate for a specific problem. Commercial codes also provide many turbulence models, but not all are suitable for all cases. The selection of a proper turbulence model for a specific case is essential for obtaining meaningful results. It is essential to critically study the results of simulation and carry out the sensitivity analysis, verification, validation and calibration, checking the robustness and reliability of the simulation. Chapter 11 deals with the sources of errors and the best practice guidelines to be followed to reduce/avoid such types of error. Analysis of results, sensitivity studies and uncertainties are provided to help CFD practitioners. The chapter also discusses the need for and methods of verification and validation.