Students' motivational beliefs about physics can influence their engagement and performance in physics as well as retention in their majors and careers. Students from underrepresented groups in physics such as women may not have enough encouragement and role models to help them develop strong motivational beliefs in physics. The societal stereotypes and biases in physics may further undermine their motivational beliefs and lead to withdrawal from physics courses or careers. In addition to motivational beliefs, students' academic performance at the end of a physics course is also an important course outcome. Prior studies have shown that factors such as students' prior preparation, quality of teaching, and sociocultural factors can influence students' motivational beliefs and academic performance. However, very few studies have investigated the effect of students' perception of the inclusiveness of the learning environment on their motivational beliefs and academic performance. In this dissertation I address the question of what role is played by students' perception of the inclusiveness of the learning environment and its relationship to gender, motivational beliefs, and academic performance in introductory physics courses. I first investigated the relationship between different motivational beliefs using both quantitative and qualitative methods. I also examined how students' physics motivational beliefs predict their identity in physics-related disciplines such as engineering. Then I analyzed gender differences in students' motivational beliefs and academic performance at the end of introductory physics courses and how different components of perception of the inclusiveness of the learning environment predict these course outcomes. These findings suggest that instructors play an important role in developing a more equitable and inclusive learning environment, in which all students can thrive. Lastly, I discuss my work on the development, validation and in-class implementation of a multiple-choice questions sequence (MQS), which was designed to help advanced undergraduate students improve their understanding of quantum measurement. [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.]