Embedded librarians working in online classrooms present a new avenue of learner support direct to students in the space where they learn. The development of embedded librarian practice has evolved since 2004 and is becoming commonplace in online tertiary classrooms, but what is not yet so common is theory underlying this new practice. A multiple case study of embedded librarians in online tertiary classrooms in New Zealand was conducted to identify what makes this practice effective in providing support. Six factors were identified as contributing to effectiveness: relationality, immediacy, relevancy, interactivity, parity and faculty--librarian collaboration. The Embedded Librarian Support model was derived from, and is presented as the culmination of, the findings of this research.