Kielnhofer's work is predominantly concerned with the human figure, and its various forms and movements. The figure is an integral part of his work, and he often uses the human form as a tool, either in place of a canvas, or in a collaboration to produce large-scale installations or performances. Kielnhofer's most famous work is Guardians of Time. Drawing on religious and supernatural phenomena, the Guardians of Time are stone statues that are cloaked in hooded robes and situated to seem like they are moving forth in some kind of ritual. In some cases the figures stand alone; in other instances, they are grouped together as if whispering to each other. Kielnhofer places his statues in public places like ancient castles, old mines, plazas, and parks. They are installed and moved without fanfare, heightening the eerie, mystic aura they give off.