Protocol analysis has become the most widely used technique for investigating design cognition. This Chapter reviews a number of protocol and other empirical studies of design activity across different domains of practice. It summarises results relevant to understanding the nature of design cognition from an interdisciplinary, domain-independent overview. Results are presented in three major aspects of design cognition—the formulation of problems, the generation of solutions, and the utilisation of design process strategies. Problem formulation includes goal analysis, solution focussing, co-evolution of problem and solution, and problem framing; solution generation includes fixation, attachment to concepts, generation of alternatives, creativity, and sketching; process strategy includes structured processes, opportunism, modal shifts, and differences between novices and experts. Parallels are drawn and comparisons made between the results, and a number of issues identified. Many similarities of design cognition across domains of professional practice are found. It seems that the ‘intuitive’ behaviour of experienced designers is often highly appropriate to design tasks, although appearing to be ‘unprincipled’ in theory.