Identification of harmonic sources in a power system has always been an important aspect in the research of power quality, which is helpful for harmonic analyzing and mitigates schemes designing. From the two aspects of PCC point harmonic current and harmonic voltage responsibility, the physical and mathematical meanings of harmonic source identification on three different levels are clearly pointed out,and the differences and similarities with Thevenin and Norton equivalent circuit models for harmonic source identification are analyzed. Quality and quantity methods and their characteristics for harmonics source identification are totally discussed and summarized, based on two different circuit models, and some research points in the future are also proposed.