Gravitational settling and microscopic diffusion affects the main sequence lifetimes, main sequence turn-off luminosities and temperatures of metal-poor halo stars. As a result, the inclusion of diffusion in the stellar evolution models has important consequences for age determinations of old stars. In contrast, diffusion has little impact on the post-main sequence evolution of metal-poor stars. Observations of the heavy element abundances in turn-off and giant branch stars in the globular cluster NGC 6397 suggest that diffusion occurs in metal-poor stars, but that the diffusion must be inhibited by turbulent mixing.