Perpendicular spin-transfer torque (p-STT) magnetic memory is gaining increasing interest as a candidate for storage-class memory, embedded memory, and possible replacement of static/dynamic memory. All these applications require extended cycling endurance, which should be based on a solid understanding and accurate modeling of the endurance failure mechanisms in the p-STT device. This paper addresses cycling endurance of p-STT memory under pulsed electrical switching. We show that endurance is limited by the dielectric breakdown of the magnetic tunnel junction stack, and we model endurance lifetime by the physical mechanisms leading to dielectric breakdown. The model predicts STT endurance as a function of applied voltage, pulsewidth, pulse polarity, and delay time between applied pulses. The dependence of the endurance on sample area is finally discussed. Index Terms-Cycling endurance, magnetic tunnel junction (MTJ), reliability analysis, reliability modeling, spintransfer torque magnetoresistive RAM (STT-MRAM). I. INTRODUCTION M AGNETORESISTIVE random access memory (MRAM) is one of the most promising memory technology due to its fast switching, nonvolatile states, high endurance, CMOS compatibility, and low current operation [1]. Thanks to these characteristics, MRAM is under intense consideration for applications as storageclass memory (SCM) [2]-[5] and embedded nonvolatile