The paper is an overview of the coding theory of the absolute photoelectric rotary encoders for angle measurement， including the absolute position coding methods and coding characteristics of reflective Gray codes， matrix codes， m-sequence codes and single-track Gray codes. The development of absolute position coding theory is based on two basic properties， uniqueness and mono-difference， and one goal to reduce the number of the tracks on the coding disc. The position coding methods introduced in this paper are reflective Gray codes with n code tracks， matrix code with n/3 tracks， m-sequence with two tracks and single-track Gray codes with only one track. However， the coding theory of single-track Gray code is not perfect， and it still relies on computer searching to obtain codes. Therefore， the fast construction method of single-track Gray codes is the development direction of absolute position coding theory in the future. Finally， the non-coding measurement method based on image sensor is introduced， which is a subversion of the traditional measurement method and is expected to become another development direction of photoelectric rotary encoders.