Company-hosted online user communities are increasingly popular as firms continue to search for ways to provide their customers with high quality and reliable support in a low cost and scalable way. Yet, empirical understanding of motivations for knowledge sharing in this type of online communities is lacking, especially with regard to an important subset of participants in these communities, the expert users. The present research attempts to narrow this gap by focusing on the motivation behind the knowledge sharing behaviors of expert-level participants in company-hosted online user communities. Semi-structured interviews were conducted using Skype, Google Hangouts, and telephone with 32 expert users and two program managers of online user communities by two well-known technology companies. Thick descriptions were used to paint a rich picture of the expert users' experiences in these company-hosted online user communities. As a result, this study revealed important detailed understanding of this subset of participants' motivation to share their knowledge with other users. According to the findings of this study, the four key factors that motivated knowledge sharing by the expert users in the company-hosted online user communities were (1) enjoyment of helping others, (2) learning, (3) passion for the hosting company and its products, and (4) affiliation. The underlying mechanisms through which these four variables functioned to drive expert-level users' knowledge sharing behavior in company-hosted online user communities were discussed. Also provided were suggestions for future research as well as recommendations for firms considering implementing a strategy of utilizing online user communities to provide authoritative, ongoing, low cost and scalable support for their customers. [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/en-US/products/dissertations/individuals.shtml.]