Atomic diffusion can have detectable effects in many kinds of stars, each timethat mixing processes are not efficient enough to prevent the build up ofabundance stratifications. In this talk, after presentation of some generalaspects about diffusion processes in stars, I will discuss the role of radiativeacceleration which is one of the important quantities determining diffusionvelocities. I will also discuss some aspects specific to stellar atmospheres, inparticular for stars exhibiting strong magnetic fields.