[en] Addressing the scaling issue is a key activity in licensing. The issue became more important when the Best Estimate Plus Uncertainty (BEPU) approach was pursued in meeting the requirements of Chapter 15 of the Final Safety Analysis Report (FSAR) of USNRC for individual NPP. Scaling is the process of demonstrating how and to what extent the numerical simulation tool validated on one or several reduced scale experiments (or at different values of some flow parameters such as pressure and fluid properties) can be applied with sufficient confidence to a real process such as an accidental scenario in a NPP. Under the lead organization and coordination of the University of Pisa, a working group has been established with the endorsement of OECD/CSNI. The group, having started its activity in December 2013, is by now finalizing the main report. A State-of-Art-Report summarizing the technical knowledge suitable for addressing the scaling issue has been prepared. The report is intended to be useful for regulators, technical safety organizations, code developers, designers and operators involved in the application of best estimate computer codes. (Author)