  We compute bounds on screened scalar field theories from hydrogen-like
systems. New light scalar fields generically have a direct coupling to matter.
Such a coupling is strongly constrained by myriad experimental measurements.
However, certain theories possess a {\it screening mechanism} that allows the
effects of this coupling to weaken dynamically, and to evade many such bounds.
We compute the perturbations to the energy levels of hydrogen-like systems due
to screened scalar fields. We then use this result in two ways. First, we
compute bounds from hydrogen spectroscopy, finding significantly weaker bounds
than have been reported before as screening effects were overlooked. Second, we
show that muonium is an intrinsically much more sensitive probe of screened
scalar fields. For chameleon models, muonium experiments probe a large part of
the parameter space that is as yet unexplored by low energy physics and has so
far only been tested by high-energy particle physics experiments.
