  The restricted-path-integral (RPI) theory of continuous quantum measurements
including the evolution of the measured systems and phenomenon of decoherence
is reviewed. The measured system is considered as an open quantum system but
without usage of any model of the measurement (of the measuring medium or the
system's environment). The propagator of a measured system (conditioned by the
measurement readout) is presented by RPI. In the important special case of
monitoring an observable the propagator and the system's wave function satisfy
Schroedinger equation with a complex Hamiltonian (depending on the measurement
readout). Going over to the non-selective description of the measurement leads
to the Lindblad master equation. In case of non-minimally disturbing
measurements this gives theory of dissipative systems avoiding difficulties of
other approaches. The whole theory is deduced from first principles of quantum
mechanics. This proves that quantum mechanics includes theory of measurements
and is therefore conceptually closed.
