Enhancing students' understanding of aspects of the nature of science (NOS) recommended by the science education community is an important goal of science instruction. This can occur only when teachers are adequately prepared for teaching NOS. However, the science education community is still considering what is needed to enhance the efficiency of NOS instruction with respect to the preparation of NOS teachers. Based on a qualitative meta-synthesis of recent literature (2009-2018), we propose a framework of competencies for what teachers need to know to be an effective NOS instructor. Fifty-eight peer-reviewed journal articles targeting NOS instruction for, and related classroom practices of, preservice and inservice teachers were analyzed. This evaluation resulted in the identification of 20 specific competencies clustered in seven categories deemed necessary in the support of effective NOS instruction, generally divided into those that are NOS specific and those directed at more general pedagogical issues. These include (1) general NOS knowledge, (2) subject matter knowledge, (3) knowledge of learners with respect to NOS, (4) knowledge about NOS instructional strategies, (5) knowledge about NOS assessment, (6) general pedagogical knowledge, and (7) teachers' motivations and beliefs about teaching NOS. The proposed competencies can be used as a guide for science educators preparing teachers who, in turn, are able to demonstrate NOS competencies and enhance the quality and depth of their students' learning.