  Cloning machines, that is, transformations that achieve the best approximate
copying of a quantum state compatible with the no-cloning theorem, have been a
fundamental research topic over the last five years. This study is of
particular significance given the close connection between quantum cloning and
quantum cryptography: using an optimal cloner generally makes it possible to
obtain a tight bound on the best individual eavesdropping strategy in a quantum
cryptosystem. In this Chapter, the issue of cloning a continuous-variable
quantum system will be analyzed, and a Gaussian cloning transformation will be
introduced. This cloner, which copies equally well any two canonically
conjugate continuous variables, duplicates all coherent states with a same
fidelity (F=2/3). The optical implementation of this cloner and its extension
to N-to-M cloners will also be discussed. Finally, the use of this cloner for
the security assessment of a quantum key distribution scheme relying on
continuous (Gaussian) key carriers will be sketched.
