  The standard definition of the electromagnetic radius of a charged particle
(in particular the proton) is ambiguous once electromagnetic corrections are
considered. We argue that a natural definition can be given within an effective
field theory framework in terms of a matching coefficient. The definition of
the neutron radius is also discussed. We elaborate on the effective field
theory relevant for the hydrogen and muonic hydrogen, specially for the latter.
We compute the hadronic corrections to the lamb shift (for the polarizability
effects only with logarithmic accuracy) within heavy baryon effective theory.
We find that they diverge in the inverse of the pion mass in the chiral limit.
