Despite being faced with major barriers to overcome, local school districts and teachers must work to align their instructional practices in science and their assessment practices to the Next Generation Science Standards (NGSS) and the New Jersey Student Learning Standards for Science (NJSLS-Science). As described in the literature, a three-dimensional approach to assessment design is one of the qualities considered the model for a true and authentic measurement of student aptitude in science. As teachers work to understand NGSS alignment and their ability to meet alignment standards as an assessor while designing and implementing NGSS-aligned assessments, they will undoubtedly encounter various factors that will either support or present a barrier to that alignment. A convergent parallel mixed methods research methodology was utilized to explore how high school science teachers perceive best assessment practices, self-concept, and ideal self regarding NGSS-aligned assessment practices. It collected closed surveys, open-ended survey responses, and interview responses, through which themes and patterns were identified in experiences related to the conceptualizations of best assessment practices, self-concept, and ideal self, as well as the perceived supports and barriers to alignment between self-concept and the ideal self. The findings suggest that teachers conceptualize best practices in science assessment as presenting opportunities for students to demonstrate science understanding by using a variety of SEPs while allowing opportunities for student choice. In addition, teachers perceive their self-concept as an assessor in science as having high levels of confidence and motivation to further develop their practices. Lastly, the perceived self-concept and ideal self-congruence experienced by teachers is supported by their use of the defined best practices in assessment. These findings have implications for future policies, practices, and research studies. [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.]