The purpose of this phenomenological study was to examine the perceptions of Texas public Montessori school principals as instructional leaders in science. Twelve public Montessori school principals were interviewed for this study. Two research questions were used: How do public Montessori principals perceive Texas science standards in public Montessori Elementary classrooms? How do principals view their role as an instructional leader in elementary science related to teachers' effectiveness and student outcomes? Research question one resulted in the following themes: (a) aligning curricula to the Texas Essential Knowledge and Skills (TEKS), (b) engaging science instruction as integrated and hands-on lessons, (c) emphasizing required district and state assessments, and (d) incorporating traditional teaching methodologies to support Montessori instruction. Research question two yielded common themes: (a) balancing Montessori methodologies and philosophies in public school settings with competing demands, (b) monitoring assessment scores as the determination of student success, (c) working in collaboration to support teacher effectiveness, and (d) providing resources and support to teachers. Implications for Montessori practitioners: paradox of Montessori education in a public school setting, strong support for science in classrooms from the principal and a need for continued research around Montessori education in public school settings. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]