  As images and spectra from ISO and Spitzer have provided increasingly
higher-fidelity representations of the mid-infrared (MIR) and Polycyclic
Aromatic Hydrocarbon (PAH) emission from galaxies and galactic and
extra-galactic regions, more systematic efforts have been devoted to
establishing whether the emission in this wavelength region can be used as a
reliable star formation rate indicator. This has also been in response to the
extensive surveys of distant galaxies that have accumulated during the cold
phase of the Spitzer Space Telescope. Results so far have been somewhat
contradictory, reflecting the complex nature of the PAHs and of the
mid-infrared-emitting dust in general. The two main problems faced when
attempting to define a star formation rate indicator based on the mid-infrared
emission from galaxies and star-forming regions are: (1) the strong dependence
of the PAH emission on metallicity; (2) the heating of the PAH dust by evolved
stellar populations unrelated to the current star formation. I review the
status of the field, with a specific focus on these two problems, and will try
to quantify the impact of each on calibrations of the mid-infrared emission as
a star formation rate indicator.
