Sacred natural places, among which hiis sites form the best known and the most thoroughly investigated part, have started to attract multifarious scientific interest only recently. Although pagan sanctuaries have been the object of research for the clergy, politicians, and historians already since the 18th century, more general and analytical studies are still lacking. The article offers a historiographical overview of sacred natural sites in Estonia and points out some aspects which play an important role in the studies. Estonian scholars have mostly avoided the subject of the sacred grove following the major study by Oskar Loorits. On the one hand it is definitely connected with condemning of religious studies in the Soviet period of scientific atheism. On the other hand, it is related to the public attitudes towards hiis and the ideological pressure of the Soviet period, which implied that hiis marked the religion of ancient and independent Estonians. Hiis sites were first associated with archaeological material in the 1990s and it has been pointed out that there is no essential connection between hiis sites and graves. Objecting to earlier interpretations, it has been suggested that hiis places have been separate sites on landscape and their initial semantic field was not necessarily connected with the dead and the burial site at all. Instead, the dominant element of landscape may have been chosen as a holy site and the latter may have been used also as a burial place. Explanation of the principles for selecting a place for the hiis has been an important topic in the past decade. Relying primarily on the concepts of holiness, anomaly, distinction from the surroundings, prominence and the presence of natural border have been stressed. A new perspective is also offered, stressing energetic field and flora anomalies. It is clear that since the majority of sources available on hiis sites is formed by undatable folk tradition, all speculations on the topic are very complicated and holy sites are perceived as something belonging to some timeless past. It is apparent that the general name hiis, holy or offering place applies to sites from different periods, some of which dating back even to the Late Bronze Age, while others might have been taken into use relatively recently.