One of the most influential descriptions of design activity emphasises how problems and solutions ‘co-evolve’. This concept has somehow escaped critical review and cross- disciplinary comparison, resulting in a fragmented approach to the subject. Reviewing the published literature on design co-evolution reveals that the term is used to refer to a range of distinct concepts, and the study of co-evolution has generated a number of elaborations and alternatives. Reviewing the broader literature in design and other disciplines further reveals that discussions of design co-evolution are disconnected from the history of relevant concepts in design research, and disconnected from a range of relevant concepts in other disciplines that describe creative work. Here I examine what the different concepts of design co- evolution are, how they have been modified and what they are related to. This leads to questioning the distinction between problems and solutions, defining them in relative terms, and drawing a connection between design co-evolution and design fixation.