This study articulates the epistemological support of reflective practices for the professional practice of crafting mathematics problems in realistic contexts. The data were collected from 15 middle school pre-service mathematics teachers in two semesters as they were generating problems and engaging in three reflective practices, which served to elicit their knowledge-in-action. The prospective teachers' reflections indicated that the knowledge-in-action for crafting realistic mathematics problems encompassed various but particular forms of the content; that is, pedagogical, curricular, and interdisciplinary forms of the content. Thus, particularly for the professional practice of crafting realistic mathematics problems, the prospective teachers identified epistemological supports that blended not only content and pedagogy but also other knowledge dimensions, which have crucial implications for mathematics teacher education.