We discuss the problem of a consistent determination of the visual extincti
on to objects in and behind a dark cloud. The use of near-infrared colours
is discussed, in particular the H-R colours. We concentrate on the uncertai
nties, with emphasis on the extinction law and on the intrinsic colours of
the young stars embedded in a dark cloud. The cases of the two intensively
studied nearby star-forming clouds, Taurus and Ophiuchus, are addressed.