  We report theoretical calculations of high harmonic generation (HHG) by
intense infrared lasers in atomic and molecular targets taking into account the
macroscopic propagation of both fundamental and harmonic fields. On the
examples of Ar and N$_2$, we demonstrate that these {\it ab initio}
calculations are capable of accurately reproducing available experimental
results with isotropic and aligned target media. We further present detailed
analysis of HHG intensity and phase, under various experimental conditions, in
particular, as the wavelength of the driving laser changes. Most importantly,
our results strongly support the factorization of HHG at the macroscopic level
into a product of a returning electron wave packet and the photorecombination
transition dipole, under typical experimental conditions. This implies that the
single-atom/molecule structure information can be retrieved from experimentally
measured HHG spectra.
