Physics education research (PER) is a new subbranch of physics that emerged in the United States. It studies how students learn physics, probes and analyses the common difficulties in learning and applying physics knowledge, examines learning gains, and evaluates various teaching technologies and methods. Based on the data acquired, theories on physics learning are developed. Under support mainly from the National Science Foundation of the USA,PER is becoming a fast growing field of scholarly research bearing fruits, which have been used in the teaching of physics in universities. Now, PER is still a new research field in China.This article aims to introduce the reader to the field of PER. Topics include its brief history,what it is, its various methods and recent developments, and its impact on college-level physics teaching. The achievements in PER in the US may be of value for the physics education reform in China.