  The Heavy Quark Expansion (HQE) has become an extremely powerful tool in
flavor physics. For charm decays, where the expansion parameters
$\alpha_s(m_c)$ and $\Lambda_{\rm QCD}/m_c$ are bigger than for bottom decays,
it remains to be seen if the HQE can be applied with similar success.
Nevertheless, to make optimal use of the plethora of data already available and
coming in the near future, a better understanding of HQE for charm decays is
crucial. This paper discusses in detail how the HQE for charm decays is set up,
what is the role of four-quark (weak annihilation) operators and how this
compares to the well understood bottom decays. Subtleties concerning radiative
corrections and the charm mass scheme are briefly discussed. An experimental
study of the relevant HQE hadronic matrix elements will then show if the HQE
expansion for charm converges well enough. Besides serving as an important
cross check for inclusive $B$ decays, in the end, this study might open the
road for inclusive $|V_{cs}|$ and $|V_{cd}|$ extractions.
