This dissertation composes three papers detailing work intended to illuminate the atomistic-scale mechanisms governing environment assisted fatigue crack growth in aluminum. All studies focus on the application of concurrent atomistic-continuum multiscale modeling, utilizing the coupled atomistic and discrete dislocation (CADD) methodology. First, an ab-initio prediction of environmental embrittlement in aluminum is demonstrated using a density functional theory (DFT) based multiscale framework to simulate the behavior of a loaded crack-tip in the presence of both elemental oxygen and hydrogen impurities. The multiscale simulations and subsequent analysis suggest that electronegative surface impurities can inhibit dislocation nucleation from a crack-tip, which is consistent with macroscopic brittle failure. Second, a series of ab-initio and multiscale simulations are performed, directly linking an atomistic mechanism of hydrogen-assisted cracking (HAC) to experimental fatigue data. The mechanism of enhanced surface deformation is demonstrated using an aluminum-only interatomic potential capable of reproducing ab-initio trends by strategically shielding critical surface bonds in accordance with the environmental exposure level. The strategic shielding approach is used within a CADD-based model to predict an embrittling effect of hydrogen on near threshold fatigue crack growth rates. Third, a CADD-based model is used to simulate the approximate effects of monolayer surface layer stiffening on near-threshold fatigue behavior in aluminum. Stiff surface layer effects are investigated by adding a Lennard-Jones overlay potential to exposed aluminum crack-face atoms. For the single crystal orientation studied, we find that deformation behavior generally begins with a short period of fast transient crack-tip propagation until a stable defect structure has accumulated ahead of the crack. Additionally, the approximate effects of stiff monolayer surface-film formation are found to inhibit crack growth by resisting typical slip-plane cracking behavior. For all studies, the results are discussed in terms of the current environment assisted fatigue literature.