Individuals in many cultures believe in omniscient (all-knowing) beings, but everyday representations of omniscience have rarely been studied. To understand the nature of such representations requires knowing how they develop. Two studies examined the breadth of knowledge (i.e., types of knowledge) and depth of knowledge (i.e., amount of knowledge within domains) that preschoolers, elementary-school children, and adults (N = 180) attributed to an all-knowing being. Preschoolers often reported that an omniscient mind would lack many types of knowledge, and they completely failed to understand the depth of omniscient knowledge. With increasing age, children approached an understanding of omniscience--attributing broader and deeper knowledge to an omniscient agent--but only adults firmly understood the depth of omniscient knowledge. We identify socio-cultural and cognitive factors that correlate with children's understandings of omniscience. Findings demonstrate that childhood representations of fallible, limited, human minds both make possible and constrain developing representations of radically non-human minds.