This paper reports the first systematic review and synthesis of protocol studies on conceptual design cognition. 47 protocol studies from the domains of architectural design, engineering design, and product de-sign engineering were reviewed towards answering the following re-search question: What is our current understanding of the cognitive processes involved in conceptual design tasks carried out by individual designers? Studies were found to reflect three viewpoints on the cognitive nature of designing, namely: design as search; design as ex-ploration; and design activities. Synthesising the findings of individual studies yielded a classification of cognitive processes involved in con-ceptual design tasks, described in different terms across different viewpoints. Towards a common terminology, these processes are posi-tioned within the cognitive psychology literature, revealing seven basic types of process that appear to be fundamental to designing across all viewpoints: memory (working and long term); visual perception; men-tal imagery; attention; semantic association; cognitive control; and higher-order processes, e.g. analysis and reasoning. The development of common cognitive models of conceptual design, grounded in a sci-entifically rigorous understanding of design cognition, is identified as an avenue for future research
