While new teacher mentoring has traditionally focused on socio-emotional support and professional socialization, understanding mentors' role in developing novices' content teaching is needed given new educational reforms. Few researchers have explored a knowledge/practice base for content-focused mentoring. Therefore, we ask: what do content mentors identify as knowledge/practices needed for subject-specific mentoring? How is subject-specific mentoring enacted? What complexities arise? We found: (a) developing novices' content teaching is a distinct mentor role; (b) a knowledge/practice base, with mentor's pedagogical content knowledge and knowledge of content-specific assessment most frequently reported; and (c) enactment of content-focused mentoring reveals promising practices in guiding novices in assessing and developing students' disciplinary thinking, and tensions between content-focused and socio-emotional mentor roles.