This article reviews the technique of verbal protocol analysis and gives a profile of its use within software engineering research over the last two decades. An overview is given of the procedures used in verbal protocol analysis, and commonly-found difficulties in the application of the technique by researchers are described. The article reports on published efforts to develop tools to automate the procedures. A review of the literature shows trends in the use of the verbal protocol analysis in software engineering research from the 1980s to the present. Recurring themes of its purpose within software engineering research are identified, including the comparison of the behaviours of subjects with differing levels of expertise and the identification of effective software comprehension strategies. Advances and problems with the development of a general-purpose encoding scheme for verbal protocol analysis appropriate to a range of domains within software engineering are described.