  The theory of quantum error correction is a cornerstone of quantum
information processing. It shows that quantum data can be protected against
decoherence effects, which otherwise would render many of the new quantum
applications practically impossible. In this paper we give a self contained
introduction to this theory and to the closely related concept of quantum
channel capacities. We show, in particular, that it is possible (using
appropriate error correcting schemes) to send a non-vanishing amount of quantum
data undisturbed (in a certain asymptotic sense) through a noisy quantum
channel T, provided the errors produced by T are small enough.
