Knowledge elicitation of the work of professional operators, using traditional methods such as concurrent or retrospective verbalization is problematic. Concurrent verbalization distracts the operators from their primary task, and the operators have difficulties in verbalizing about their automated work tasks. Retrospective verbalization on the other hand, suffers from rationalization problems. An operator might give a perfectly good explanation of some action taken and might also be completely confident about truth of the verbalized information, when it in fact is incorrect. To overcome some of these problems, this thesis presents a new complementary verbalization method called Collegial Verbalization (CV). The CV-method utilizes the shared knowledge among work colleagues to improve the quality of the resulting information. The method consists roughly of the following steps; (1) Video tape subjects while they are working. (2) Play back interesting events to the subject’s colleagues individually and let them verbalize on the subject’s actions. (3) Compare the colleagues’ verbal reports to each other to find similarities, differences, etc. Throughout my research I have formulated, defined and assessed the new method in detail. The method has been applied to study train drivers, high-speed ferry operators, train traffic dispatchers and the medical staff at intensive care units. Comparative studies have shown; (1) that CV-protocols can be used as an independent source of data, (2) that colleagues produce reports with similar characteristics of retrospective verbal reports, (3) that the CV-method can produce more information than retrospective verbalization, because of the advantage of using multiple narrators. When the intention is to gather data as input to design, rather than establishing the original thought processes form the time of the studied events, the CV-method can also produce more reliable information than retrospective verbalization, because of the advantage of using multiple narrators. Based on these results, I have concluded that the CV-method has a clear advantage as a complementary information acquisition method, when studying the work of professional operators. The thesis ends with a discussion about several additional possible applications for the CV-method, such as applied team learning or psychological research in the field of decision making.