  Quantum information processing rests on our ability to manipulate quantum
superpositions through coherent unitary transformations. In reality the quantum
information processor (a linear ion trap, or cavity qed implementation for
example) exists in a dissipative environment. Dephasing, and other technical
sources of noise, as well as more fundamental sources of dissipation severely
restrict quantum processing capabilities. The strength of the coherent coupling
needed to implement quantum logic is not always independent of dissipation. The
limitations these dissipative influences present will be described and the need
for efficient error correction noted. Even if long and involved quantum
computations turn out to be hard to realize, one can perform interesting
manipulations of entanglement involving only a few gates and qubits, of which
we give examples. Quantum communication also involves manipulations of
entanglement which are simpler to implement than elaborate computations. We
briefly analyse the notion of the capacity of a quantum communication channel.
