This article seeks the medieval definition of Sukuyōdō by looking at those activities recorded in the Azuma Kagami that were specifically attributed to sukuyōshi, primarily apotropaic rituals to astral deities. These astral deities were found in the retinue or identified with various buddhas and bodhisattvas, and thus can be found in non-Sukuyōdō Buddhist rites as well, showing that particular astral deities were not the means by which Sukuyōdō was marked as a separate field. Through a close reading of complex ritual programs, as well as use of the concepts of “boundary-work” and “boundary objects,” taken from the sociology of science, this article shows how Sukuyōdō was seen as auxiliary to esoteric Buddhism at the developing Kamakura court, and that the definition of Sukuyōdō in the thirteenth century depended onspecific types of  rituals, and not the object of devotion in those rituals.