In her paper, Conservation as Psychology, Jane Grenville [(2007). “Conservation as Psychology: Ontological Security and the Built Environment.” International Journal of Heritage Studies 13 (6): 447–461. DOI:10.1080/13527250701570614] proposed that the built historic environment plays a key role in the development of individual ontological security. In view of the need to better understand the unique wellbeing effects of the historic environment, the current paper explores this theory further with reference to the prehistoric archaeology of the Stonehenge and Avebury WHS, the Vale of Pewsey and their environs in Wiltshire, UK. Considering the qualitative findings of research undertaken in these prehistoric landscapes from the theoretical perspective underpinning Grenville’s work, this paper suggests that they have the capacity to impact ontological security, and thus existential wellbeing, in a significant way.