Supporting students at all levels in and out of the classroom is an important goal for physics education. In this thesis, the first theme focuses on conductors and insulators and the effectiveness of a guided inquiry-based tutorial that uses research on student difficulties as a guide to improve student understanding in an algebra-based introductory physics course at a large research university. Student difficulties were investigated by administering written surveys to students and conducting individual interviews with students. The tutorial acts as a scaffolding tool and provides students opportunities to strengthen their understanding of these concepts. The effectiveness of the tutorial was measured using a post-test. The next theme broadens the scope to four introductory physics courses for science and engineering majors and bio-science and health-related majors to determine the characteristics of students who repeat introductory physics to get insight into possible ways to help them. This study was conducted using data spanning 13 years at a predominantly White, large research institution (LRI). The last theme focuses on investigations employing a qualitative approach to understand inequities in physics learning environments using traditionally marginalized students' experiences in physics and astronomy college programs. To understand the experiences of women in undergraduate and graduate physics and astronomy programs, we conducted semi-structured empathetic interviews. The first study was conducted with graduate women of color in physics at a predominantly White LRI, the second with undergraduate women at a predominantly White LRI, and the third with undergraduate women in a medium-sized physics department at a predominantly White small liberal arts college. Lastly, we present a comparative case study of the physics cultures in three departments, including the second and third set of interviews and a study conducted by Angela Johnson.