Information literacy describes expertise in information problem-solving. This expertise includes facility in several endeavors addressed by the information behavior literature, including information needs, seeking, and use. Definitions and descriptions of information literacy suggest that this expertise is broadly applicable to a variety of information problems in a variety of content domains. In the library and information science literature, there is scant coverage of the information problem-solving experts, how they think, or whether this expert thinking is transferable to new situations and new contexts. The literature of cognate fields suggests that experts in a variety of domains are more effective in solving problems within their domain than novices because they are more successful in the initial stage of the process, where problem situations are categorized according to those successfully resolved in past experiences. This research project addresses this gap in the library and information science literature by examining the nature and impact of the initial stage of the information problem-solving process, termed Task Definition by the Big6 approach. More than two hundred participants completed a series of online triad judgment tasks designed to identify differences in categorical determinations made by experts and novices in the domain of librarianship, the nature and impact of these differences, and whether these differences extend across domains. In a follow-up investigation, thirty participants met with the researcher and were given similar sorting tasks; verbal reporting of their personal constructs was recorded via Kelly's (1955) repertory grid (RepGrid) technique. The findings indicate that training and experience in librarianship produce predictable expert behavior in information problem-solving, that this expertise is appropriately described by the expertise literature of cognate fields, and that the expert thinking of librarians has value in multiple contexts. This research contributes to a conceptual understanding of information literacy by highlighting the crucial stage of Task Definition in the information problem-solving process, expands the current repertoire of methods currently used to investigate human information behavior, and informs the pedagogy of information literacy as practiced in schools, libraries, and work environments. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]