This joint session between FM21 and FM2 (“Astronomical Heritage: Progressing the UNESCO–IAU Initiative”) focused upon the need to preserve the dark skies necessary for the continued functioning of the world's leading optical observatories and whether, if some of the sites concerned could be inscribed on UNESCO's World Heritage List, this could help achieve this objective. Among the main issues addressed were: is a WHL inscription feasible in the first place? how could the strongest case for inscription be made? what progress has been made towards doing this? and what other effects might a WHL inscription have and would they all be desirable to astronomers? Addressing such issues involves not only scientific but also heritage and political considerations.