This study is based on a Delphi study on environmental literacy which is related to both teachers professional development and environmental science, technology, engineering, and mathematics ( E+ STEM ) literacy. In the light of the expert opinions, the goal is to determine what teachers should do to develop their experiences and qualifications as E+ STEM literate individuals. In this study, a "mixed method" research design, in which both qualitative and quantitative methods are involved, is used to reveal the expert opinions. The exploratory sequential design, which is one type of mixed method research, is used. In the first step of the Delphi study, qualitative data are collected about teachers' professional development. After analyses of data in the first step of Delphi study, the quantitative form is developed for second step of the Delphi study. Finally, after analyses of the data in the second step, the final quantitative form (3 rd step) is prepared again. It is performed in three consecutive steps in Delphi study. The sample consists of the 45 experts who initially accepted to participate in the study. 20 of the 45 experts participated in the first step Delphi. The number of participants in the second and third Delphi study, respectively, is 44 and 26, respectively. It is concluded that there is a consensus about "having and updating content knowledge about environmental issues," "following the development of environmental technologies, and applying them in class." There is additional agreement about "having and developing pedagogical competencies for the development of teachers" experiences and qualifications as E+ STEM literate individuals. It is suggested that the concept of "environment" should be integrated into the framework of "STEM pedagogical content knowledge (PCK)" for teacher's professional development. By this means, a new educational and environmental concept, E+ STEM-PCK, would be incorporated in teacher education.