Background: Atrial tachycardia (AT) occurring following catheter ablation of persistent atrial fibrillation (AF) may be challenging to map and ablate because their mechanism and location is unpredictable and may be multiple in an individual patient. Methods and Results: A prospective cohort of 128 consecutive patients presenting 246 AT in the context of prior AF ablation was investigated. Using activation and entrainment mapping and applying the consensus definition of AT, we evaluated a deductive diagnostic approach based on up to three steps: (1) cycle length regularity, (2) search for macroreentry (i.e., involving >2 separate atrial segments), and (3) if macroreentry excluded, search for focal origin giving a centrifugal activation of the atria. A total of 238/246 (97%) sustained AT (mean cycle length [CL] 284 ± 87 ms) were successfully mapped (single AT, 51 pts; multiple AT, 77 pts) with a diagnostic time of 10 ± 8 min per tachycardia. AT were macroreentrant in 109 (46%) and focal in 129 (54%). Of the latter, only 34 focal AT originated from a discrete point site fulfilling the consensus criteria, while a distinct mechanism, localized reentry (AT that was neither macro reentry nor focal), was identified in 95. Localized reentry was defined by (1) electrograms covering ≥75% of the cycle length of AT within an area covering a single or 2 contiguous segments, (2) postpacing interval (PPI) <30 ms at the site, (3) an identifiable zone of slow conduction, and (4) centrifugal activation of the atrium from the area. Conclusions: This prospective study demonstrates the feasibility of rapid and accurate identification of all types of postablation AT in a large cohort of patients and describes the dominant role of localized reentry as a novel mechanism of AT.