  Neutron stars are compact and dense celestial objects that offer the unique
opportunity to explore matter and its interactions under conditions that cannot
be reproduced elsewhere in the Universe. Their extreme gravitational,
rotational and magnetic energy reservoirs fuel the large variety of their
emission, which encompasses all available multi-messenger tracers:
electromagnetic and gravitational waves, neutrinos, and cosmic rays. However,
accurately measuring global neutron-star properties such as mass, radius, and
moment of inertia poses significant challenges. Probing internal
characteristics such as the crustal composition or superfluid physics is even
more complex. This article provides a comprehensive review of the different
methods employed to measure neutron-star characteristics and the level of
reliance on theoretical models. Understanding these measurement techniques is
crucial for advancing our knowledge of neutron-star physics. We also highlight
the importance of employing independent methods and adopting a multi-messenger
approach to gather complementary data from various observable phenomena as
exemplified by the recent breakthroughs in gravitational-wave astronomy and the
landmark detection of a binary neutron-star merger. Consolidating the current
state of knowledge on neutron-star measurements will enable an accurate
interpretation of the current data and errors, and better planning for future
observations and experiments.
