Do you know the extinction in your young massive cluster?
  Up to ages of ~100 Myr, massive clusters are still swamped in large amounts
of gas and dust, with considerable and uneven levels of extinction. At the same
time, large grains (ices?) produced by type II supernovae profoundly alter the
interstellar medium (ISM), thus resulting in extinction properties very
different from those of the diffuse ISM. To obtain physically meaningful
parameters of stars, from basic luminosities and effective temperatures to
masses and ages, we must understand and measure the local extinction law. This
problem affects all the massive young clusters discussed in his volume. We have
developed a powerful method to unambiguously determine the extinction law in an
uniform way across a cluster field, using multi-band photometry of red giant
stars belonging to the red clump (RC). In the Large Magellanic Cloud, with
about 20 RC stars per arcmin^2, we can easily derive a solid and
self-consistent absolute extinction curve over the entire wavelength range of
the photometry. Here, we present the extinction law of the Tarantula nebula (30
Dor) based on thousands of stars observed as part of the Hubble Tarantula
Treasury Project.
