  Effective field theories are the most general tool for the description of low
energy phenomena. They are universal and systematic: they can be formulated for
any low energy systems we can think of and offer a clear guide on how to
calculate predictions with reliable error estimates, a feature that is called
power counting. These properties can be easily understood in Wilsonian
renormalization, in which effective field theories are the low energy
renormalization group evolution of a more fundamental ---perhaps unknown or
unsolvable--- high energy theory. In nuclear physics they provide the
possibility of a theoretically sound derivation of nuclear forces without
having to solve quantum chromodynamics explicitly. However there is the problem
of how to organize calculations within nuclear effective field theory: the
traditional knowledge about power counting is perturbative but nuclear physics
is not. Yet power counting can be derived in Wilsonian renormalization and
there is already a fairly good understanding of how to apply these ideas to
non-perturbative phenomena and in particular to nuclear physics. Here we review
a few of these ideas, explain power counting in two-nucleon scattering and
reactions with external probes and hint at how to extend the present analysis
beyond the two-body problem.
