The purpose of this exploratory descriptive research was to examine what Association Montessori Internationale (AMI) certified Montessori teachers (MTs) for students ages 3-6 years in the United States reported regarding scientific observation (SO) as a disposition and as a disposition in action (DIA) to better understand MTs' application of scientific observation for assessment (SOFA). A national survey with an embedded mixed methods case study was employed using a single-phase convergent mixed-methods design which supported the collection of quantitative and qualitative data. The data were analyzed by initiative and merged for interpretation. The embedded case study served to pilot a three-pronged approach as a sound methodology for the examination of teacher reports regarding SO as a disposition and DIA: survey, teacher observation and interview. The instrumentation integrates the Montessori "trinity" (Montessori, 1989) with the fundamental characteristics of SO and the conceptual framework consisting of dispositions and the theory of DIA (Thornton, 2006). While MTs with the AMI teacher credential for students ages 3-6 in the United States understood the importance of SOFA for teacher practice in a Montessori classroom for children ages 3-6 years, aspects of SO DIA were not evident. In addition, the MTs reported the application of SOFA existed with some degree of predefined aims and objectives, and was related to system challenges for developing and sustaining SO procedures, as well as collegial dissemination when time and organizational constraints permitted. Recommendations for practice include explicit instruction with formative and summative assessment using SO during preservice teacher training along with coaching Montessori school leadership to identify obstacles to this MT professional practice and to develop schoolwide expectations for the application of SOFA. Future qualitative research to examine organizational constraints and systems that effect the application of SOFA would support MT professional practice. Additionally, a future mixed method, multiple case study using the observational protocol with DIA rubric: SO and interviews to explore the range of SO DIA would benefit teacher preparation programs. [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.]