Search expertise has long been studied and used extensively in information seeking behavior research, both as a fundamental concept and as a method of comparing groups of users. Unfortunately, while search expertise has been studied for some time, the conceptualization of it has lagged behind its use in categorizing users. This has led to users who were defined as experts in one study who could be considered novices in another study. Not only does this make it difficult to know how search expertise impacts the issues being studied, it also makes it difficult to compare results between studies. It is clear that search expertise is more important now than ever as the information and misinformation available online grows exponentially. It must first be conceptually designed and modeled, and then it must be operationalized so that it can be reliably measured. This research first examined prior research related to online search expertise and created a working definition and model. One-on-one interviews were then conducted with nine known search experts, who were asked to describe online search expertise. These same experts were then gathered into three separate focus groups where they examined and grouped the items gathered from their individual interviews. The items and groupings from the focus groups were then used to update the model and also to create an initial instrument to measure online search expertise. That initial instrument was then given to 14 targeted participants in one-on-one cognitive interviews. The instrument was modified based upon the results of those interviews and then given to four targeted groups of participants and the results from 466 of those participants were examined using statistical methods. The results support the use of aggregate scores for past experience, self-rated search ability, and search skill ability as continuous measures of online search expertise. While the personality items used in the research suffered from the same inconclusive results as prior research, the inclusion of analytical abilities in future versions is indicated. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]