This paper tracks a debate that occurred, first, within the field of Ubiquitous Computing but quickly spread to CHI and beyond, in which design scholars argued that seamlessness had long been an implicit and privileged design virtue, often at the expense of seamfulness. Seamless design emphasizes clarity, simplicity, ease of use, and consistency to facilitate technological interaction. Seamful design emphasizes configurability, user appropriation, and revelation of complexity, ambiguity or inconsistency. Here we review these literatures together and argue that, rather than rival approaches, seamful and seamless design are complements, each emphasizing different aspects of downstream user agency. Ultimately, we situate this debate within the larger, perennial discussion about the strategic revelation and concealment of human and technological operations and therein the role of design. CCS CONCEPTS • Human-centered computing → HCI theory, concepts and models.