Single-phase Computational Fluid Dynamics (CFD) is used more and more for design and safety issues related to Light-Water Reactor (LWR) thermal hydraulics. Over the past ten years, the Working Group for the Analysis and Management of Accidents (WGAMA) has initiated activities to promote the use of CFD for Nuclear Reactor Safety (NRS). A list of safety issues for which CFD may bring real benefits was established. Best practice guidelines (BPGs) applicable to single phase CFD were written. Assessment requirements were also addressed in a report with particular attention to a few safety issues. These past activities provided more confidence in the application of CFD for safety by defining the conditions and requirements for having some confidence in the predictions. However, no applicable methods have been published about a possible quantitative evaluation of the uncertainty of predictions, and such an evaluation is mandatory for complementing a best estimate approach within a nuclear reactor licensing framework. Thus, a review of the methodologies for determining the uncertainty of CFD predictions applied to reactor thermal hydraulics was initiated. This is a very recent area of investigation, and the reported activity is rather limited. Only a few prospective works are in progress. One must first list what exists in order to conclude what needs remain. A comparison with system codes may be useful since available uncertainty methods for system codes are rather mature as the BEMUSE project (NEA/CSNI/R(2011)4) has shown.