  The delay time associated with a scattering process is one of the most
important dynamical aspects in quantum mechanics. A common measure of this is
the Wigner delay time based on the group velocity description of a wave-packet,
which my easily indicate super-luminal or even negative times of interaction
that are unacceptable. Many other measures such as dwell times have been
proposed, but also suffer from serious deficiencies, particularly for
evanescent waves. One important way of realising a timescale that is causally
connected to the spatial region of interest has been to utilize the dynamical
evolution of extra degrees of freedom called quantum clocks, such as the spin
of an electron in an applied magnetic field or coherent decay or growth of
light in an absorptive or amplifying medium placed within the region of
interest. Here we provide a review of the several approaches developed to
answer the basic question - how much time does a quantum particle (or wave)
spend in a specified region of space? While a unique answer still evades us,
important progress has been made in understanding the timescales and obtaining
positive definite times of interaction by noting that all such clocks are
affected by spurious scattering concomitant with the very clock potentials,
however, weak they be and by eliminating the spurious scattering.
