  In this paper, we review the use of parity as a detection observable in
quantum metrology as well as introduce some original findings with regards to
measurement resolution in Ramsey spectroscopy and quantum non-demolition (QND)
measures of atomic parity. Parity was first introduced in the context of Ramsey
spectroscopy as an alternative to atomic state detection. It was latter adapted
for use in quantum optical interferometry where it has been shown to be the
optimal detection observable saturating the quantum Cram\'{e}r-Rao bound for
path symmetric states. We include a brief review of the basics of phase
estimation and the connection between parity-based detection and the quantum
Fisher information as it applies to quantum optical interferometry. We also
discuss the efforts made in experimental methods of measuring photon-number
parity and close the paper with a discussion on the use of parity leading to
enhanced measurement resolution in multi-atom spectroscopy. We show how this
may be of use in the construction of high-precision multi-atom atomic clocks.
