Using Social Network Analysis to Guide Theoretical Sampling ties serving specific professional groups; most dealing with computer topics, but many others addressing fields far removed from computers, such as taxation issues (misc.taxes.moderated), medical transcription (sci.med.transcription) or farming (uk.business.agriculture). This spontaneous forming of online communi- ties explains why Usenet was an early platform for conducting unobtrusive naturalistic research of computer-mediated communications and the social environments that emerge from them (Lee, 2000). Since newsgroup messages are posted online for all to see, they constitute a publicly accessible record of discussions, offering a wealth of research data. A substantial portion of the extant literature about virtual communities is focused on Usenet newsgroups and mailing lists or listservs, a similar technology. As for the newer Web 2.0 platforms, research about virtual communities is fairly recent, with blogs taking the lead. Various to focus ethnographic analysis on this purposive sample. This chapter describes the steps taken for theoretical sample selection in an ongoing eth- nography of the newsgroup misc.taxes.moderated (henceforth MTM) which hosts a long established community of practice (CoP) of tax professionals. The ethnography aims to provide a rich descrip- tion of day-to-day interactions in the community. Theoretical sampling of relevant interactions was guided by a specific SNA technique, the continu- ous core-periphery model (Borgatti & Everett, 1999). The chapter describes the procedure used for data collection, social network analysis, identi- fication of core members and theoretical sampling of discussion threads dominated by core members. The chapter is organized as follows. Section 1 provides background research on virtual commu- nities and applications of SNA to Usenet. Section 2 explains how participants form social networks