  The European gamma-ray observatory INTEGRAL has found a group of hard X-ray
sources which are highly absorbed, i.e., with column densities higher than
about 10^{23}/cm^2. Here I give an overview of this class of INTEGRAL sources.
The X-ray, as well as the optical/IR, properties of these sources and their
location in the sky suggest that they belong to the class of high-mass X-ray
binaries, some of them possibly long-period X-ray pulsars. The donors in these
binaries are most probably giant or supergiant stars. I suggest that the soft
X-ray spectrum below ~5 keV of IGR J16318-4848, as well as in several other
X-ray binaries (e.g., XTE J0421+56), can be described by emission from a
compact object which is strongly absorbed by a partionally ionised dense
envelope.
