  Decoherence of a quantum system (which then starts to display classical
features) results from the interaction of the system with the environment, and
is well described in the framework of the theory of continuous quantum
measurements (CQM). Reviewed are the various approaches to the CQM theory, and
the approach based on the effective complex Hamiltonians is discussed in
greater detail. The effective complex Hamiltonian is derived from the
restricted path integral, which emphasizes the role of information in the
dynamics of the system being measured. The complex Hamiltonian is used for
analyzing the CQM of energy in a two-level system. Such measurement is
demonstrated to be capable of monitoring the quantum transition, and the back
effect of monitoring on the probability of transition is analyzed. The
realization of this type of measurement by a long series of soft observations
of the system is presented.
