This paper describes 3D tomographic investigations of the structural evolution of Ni-yttria-stabilized zirconia (Ni-YSZ) and (La,Sr)MnO3-YSZ (LSM-YSZ) composite solid oxide fuel cell (SOFC) electrodes. Temperatures higher than normally used in SOFC operation are utilized to accelerate electrode evolution. Quantitative 3D FIB-SEM and X-ray tomographic imaging contributes to development of mechanistic evolution models needed to accurately predict long-term durability. Ni-YSZ anode functional layers annealed in humidified hydrogen at 900–1,100 °C exhibited microstructural coarsening leading to a decrease in three-phase boundary (TPB) density. There was also a change in the fraction of pores that were isolated, which impacted the density of electrochemically active TPBs. The polarization resistance of optimally fired LSM-YSZ electrodes increased upon thermal aging at 1,000 °C, whereas that of under-fired electrodes decreased upon aging. These results are explained in terms of observed 3D microstructural changes.