This paper reports on recent progresses in modeling bi-polar RRAM devices based on hafnium oxide. The unique modeling environment adopted for the simulation of device operations accounts self-consistently for the charge and ion transport, and the structural device modification occurring during forming and set/reset operations. Reliability mechanisms as well as the major sources of devices variability are included thanks to a multi-scale approach that connects the electrical device performance to the atomic-level material properties. The modeling methodology can be successfully applied to both improve device performances and fabrication process of state-of-the-art RRAM devices, and devise device solutions for future 3D RRAM architectures.