For many years, there have been discrepancies between masculine and feminine populations in science, technology, engineering, and mathematics (STEM) fields. The problem was the gender disparity in middle and high school STEM classrooms, in which teachers often lack an understanding of the underlying reasons and measures needed to lessen the gender gap. The purpose of the basic qualitative study was to explore central New Jersey middle and high school teachers' perceptions of the gender gap in STEM classrooms to determine possible means of lessening the gap. Most of the literature pertaining to the gender gap was gathered from the perspective of students and their feelings towards the gender gap, so this research literature aimed to close the gap by understanding perceptions from STEM teachers' perspectives. The gender schema theory helped to analyze research data and answer the research questions. Two research questions were developed to unveil: (a) teachers' perceptions about the gender gap in STEM, and (b) the strategies used to address the gender gap and engage girls in STEM. A sample size of 23 STEM teachers was used to collect qualitative data via open-ended questionnaires and semi-structured interviews. Thematic analysis divulged six themes including confidence levels, stereotypes, maturity and motivation, external influences, building comfort and confidence, and female role models and representation. Results expressed that teachers have gendered perceptions about girls in STEM, and there is a recommendation to improve teaching approaches in STEM so that educators can effectively engage girls in STEM and narrow the gender gap. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]