This paper examines the role of facilitation in the Investigations Online seminars, which support the professional learning of elementary school teachers and mathematics specialists using the Investigations in Number, Data, and Space curriculum. Our study considered how a facilitator fosters collaborative discussion that leads to co-construction of knowledge among seminar participants. We used a mixed methodology, incorporating both quantitative and qualitative approaches. Based on a technique for analyzing text messages in online courses, we first assessed levels of teaching presence (Anderson, Rourke, Garrison, & Archer, 2001) through a comparative analysis of six Web-based seminars. Although facilitation in online professional development shares characteristics of teaching presence in online courses, we observed that professional learning entails the use of additional facilitation strategies. To further investigate the role of facilitation in the context of online discussions, we adopted a grounded theory methodology. Taking examples from these discussions, this paper describes a key strategy, "steering toward others," that the facilitator uses to foster mathematical inquiry and build professional knowledge for teaching practice. Appended are: (1) Profile of the Online Facilitator; (2) Teaching Presence; (3) Initial Categories From Open Coding; and (4) Refined Coding Scheme. (Contains 2 tables.)