This study is an attempt to gain new insight, on behalf of science teachers, into the integration of metacognition (MC) into science education. Participants were 44 elementary school science teachers attending an in-service teacher-training (INST) program. Data collection was carried out by several data sources: recordings of all verbal discussions that took place during the program, teachers' written reflections, and semi-structured individual interviews. Our study provides a qualitative analysis of the 44 teachers' voices as a group, as well as a detailed case-study narrative analysis of three teachers' stories The findings show that the teachers' intuitive (pre-instructional) thinking was incomplete and unsatisfactory and their voices were skeptical and against the integration of MC. After teachers had mastered the notion of MC in the INST program, the following outcomes have been identified: (a) teachers expressed amazement at how such an important and relevant issue had been almost invisible to them; (b) teachers identified the affective character of metacognitive experiences as the most significant facet of MC, which acts as a mediator between teaching and learning; (c) the complete lack of learning materials addressing MC and the absence of supportive in-classroom guidance were identified as the major obstacles for its implementation; (d) teachers expressed a willingness to continue their professional development toward expanding their abilities to integrate MC as an inseparable component of the science curriculum. The implications of the findings for professional development courses in the field of MC are discussed.