Alzheimer disease is characterized by neurofibrillary pathology containing paired helical filaments (PHF). These abnormal filaments consist of a modified form of microtubule associated protein tau. The modification involves phosphorylation. In this mini review, we summarize recent studies regarding the differences between normal tau and PHF-tau, focusing especially on the extent and the site of phosphorylation. We also discuss the mechanisms possible involved in the development of PHF.