  The pion-deuteron scattering length is computed to next-to-next-to-leading
order in baryon chiral perturbation theory. A modified power-counting is then
formulated which properly accounts for infrared enhancements engendered by the
large size of the deuteron, as compared to the pion Compton wavelength. We use
the precise experimental value of the real part of the pion-deuteron scattering
length determined from the decay of pionic deuterium, together with constraints
on pion-nucleon scattering lengths from the decay of pionic hydrogen, to
extract the isovector and isoscalar S-wave pion-nucleon scattering lengths, a^-
and a^+, respectively. We find a^-=(0.0918 \pm 0.0013) M_\pi^{-1} and
a^+=(-0.0034 \pm 0.0007) M_\pi^{-1}.
