Stellar evolution models of a 0.8 Msolar star (Y=0.2352, Z=0.0001, ?=0.3) have been calculated in detail from the zero-age main sequence to the horizontal branch, both with and without atomic diffusion. We analyze the effects of atomic diffusion on the horizontal branch models themselves, as well as on the properties of giant-branch tip stars that determine the location of horizontal branch stars on the H-R diagram. It leads to a He core mass increase of 0.0025 Msolar. There are also concentration differences of up to ~0.04 dex for most metals between the He core and the exterior to the core. It is generally assumed in evolution calculations of horizontal branch stars that overshooting plays an important role in ensuring convective neutrality at the helium-burning core boundary, causing a 50% increase in the mass of the convective core. It is shown here that atomic diffusion can play this role, although a contribution from overshooting is not excluded. Atomic diffusion is also shown to lead to some hydrogen burning within the He core. Some hydrogen diffuses toward the center and leads to a CN cycle involving C produced at the time of the He flash. This increases the zero-age horizontal branch luminosity. The sensitivity to thorough mixing of the He core is discussed. The cumulative effect is to lead to a luminosity difference of 0.015+/-0.005 dex in zero-age horizontal branch stars between models with and without diffusion. This will have a small effect on the ages of globular clusters if their distances are based on the predicted luminosities of HB stars.