This paper discusses the research on three questions to improve physics instruction: (1) What is involved in understanding and using physics? (2) What do students bring to physics classes? and (3) How do students respond to physics instruction? At the heart of physics education research is a shift in emphasis on: "What are the students learning and how do we make sense of what they do?" In an effort to find out what student's real difficulties are, physics education researchers use a variety of tools. One task is to determine the range of most common possibilities called, "state space." A way to accomplish the task is to interview a number of students, letting them describe what they think about a particular situation or having them work through a problem. The researcher encourages the students to "think aloud" and to explain their reasoning. The goal is not to help the students come up with the "correct" answer but rather to understand their thinking. Interviews are videotaped, transcribed, and analyzed by multiple researchers. After a good understanding of student confusions and difficulties is obtained, multiple choice tests of surveys can be carefully designed, with the distractors selected from common spontaneous wrong answers. Discipline-based education research and components of problem solving, concepts, and expectations are discussed. Building and evaluating research-based curricula is also highlighted. (ASK/AA)