  Depending on mass and rotational frequency, gravity compresses the matter in
the core regions of neutron stars to densities that are several times higher
than the density of ordinary atomic nuclei. At such huge densities atoms
themselves collapse, and atomic nuclei are squeezed so tightly together that
new particle states may appear and novel states of matter, foremost quark
matter, may be created. This feature makes neutron stars superb astrophysical
laboratories for a wide range of physical studies. And with observational data
accumulating rapidly from both orbiting and ground based observatories spanning
the spectrum from X-rays to radio wavelengths, there has never been a more
exiting time than today to study neutron stars. The Hubble Space Telescope and
X-ray satellites such as Chandra and XMM-Newton in particular have proven
especially valuable. New astrophysical instruments such as the Five hundred
meter Aperture Spherical Telescope (FAST), the square kilometer Array (skA),
Fermi Gamma-ray Space Telescope (formerly GLAST), and possibly the
International X-ray Observatory (now Advanced Telescope for High ENergy
Astrophysics, ATHENA) promise the discovery of tens of thousands of new
non-rotating and rotating neutron stars. The latter are referred to as pulsars.
This paper provides a short overview of the impact of rotation on the structure
and composition of neutron stars. Observational properties, which may help to
shed light on the core composition of neutron stars--and, hence, the properties
of ultra-dense matter--are discussed.
