In this paper, we discuss the use of spontaneous photon emission microscopy (PEM) for observing filaments formed in HfO 2 resistive random access memory (ReRAM) cells. The setup employs a CCD and an InGaAs camera, revealing photon emissions in both forward ( set ) and reverse ( reset ) bias conditions. Photon emission intensity is modeled using an electric-field equation and inter-filament distance and density are determined assuming a uniform spatial distribution. The paper also discusses the use of high frame rate and prolonged photon emission measurements to assess lifetime and reliability and explains how single filament fluctuations and multiple filaments in a single cell were observed for the first time.