Biomarkers have revolutionized the study and clinical diagnosis of Alzheimer’s disease (AD). While amyloid-β accumulation begins decades before the onset of clinical dementia in AD, tau pathology is more closely associated in both space and time to neurodegeneration and to clinical dysfunction. Correspondingly, tau-PET may prove useful in determining the severity of AD. Building on the biological research framework for AD, we review here methods and rationale to stage the severity of AD in vivo using the topographical distribution of tau-PET. We discuss how tau-PET can be used to detect early and subthreshold tau accumulation in medial temporal cortices prior to the onset of cognitive symptoms. Furthermore, tau-PET can be used to monitor the severity of AD as tau-PET spreads to association cortices and finally primary sensory cortices. We discuss the utility of tau-PET to monitor the progression of AD, the flexibility of potential approaches, and applications for clinical trials. In this regard, topographical information from tau-PET is a useful addition to the A/T/(N) framework.