  We consider the most accurate tests of bound state QED theory of the
hyperfine splitting in two-body atoms related to the HFS interval of the 1s
state in muonium and positronium and the 2s state in hydrogen, deuterium and
the helium-3 ion. We summarize their QED theory and pay special attention to
involved effects of strong interactions and to recent optical measurements of
the 2s HFS interval in hydrogen and deuterium. We present results for specific
ratios of the 1s-2s frequencies in hydrogen and deuterium which happen to be
among the most accurately measured and calculated quantities.
