Time-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of ∼1.3 ns, a 100 × 300 nm 2 ellipsoidal device switches in ∼1 ns via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods. Spin-polarized current-induced magnetization switching (CIMS) has now been reported in many experimental works involving a wide variety of geometries including point contacts, nanopillars (spin valves or tunnel junctions), and nanowires with or without notches. 1 These systems are extensively studied, in part, because they hold the potential for applications in spin-transfer magnetic random access memory. 2 Interest in materials with perpendicular magnetic anisotropy (PMA) has grown considerably as a pathway for lowering the critical current required to switch the magnetization while maintaining thermal stability as compared with in-plane systems. 3,4 This interest has, to a large degree, stemmed from calculations of the switching behavior using a macrospin approximation. 5