  We review recent determinations of the present day and initial mass functions
in various components of the Galaxy, disk, spheroid, young and globular
clusters. As a general feature, the IMF is well described by a power-law form
for $m\ga 1 \msol$ and a lognormal form below. The extension of the disk IMF
into the brown dwarf (BD) regime is in good agreement with observations and
yields a disk BD number-density comparable to the stellar one $\sim 0.1 \pc3$.
The IMF of young clusters is found to be consistent with the disk field IMF,
providing the same correction for unresolved binaries. The spheroid IMF relies
on much less robust grounds. Within all the uncertainties, it is found to be
similar to the one derived for globular clusters, and is well represented also
by a lognormal form with a characteristic mass slightly larger than for the
disk. The IMF characteristic of early star formation remains undetermined, but
different observational constraints suggest that it does not extend below $\sim
1 \msol$. These IMFs allow a reasonably robust determination of the Galactic
present-day and initial stellar and brown dwarf contents. They also have
important galactic implications in yielding more accurate mass-to-light ratio
determinations. The M/L ratios obtained with the disk and the spheroid IMF
yield values 1.8 and 1.4 smaller than a Salpeter IMF, respectively. This
general IMF determination is examined in the context of star formation theory.
(shortened)
