  A new era in fundamental physics began when pulsars were discovered in 1967.
Soon it became clear that pulsars were useful tools for a wide variety of
physical and astrophysical problems. Further applications became possible with
the discovery of the first binary pulsar in 1974 and the discovery of
millisecond pulsars in 1982. Ever since pulsars have been used as precise
cosmic clocks, taking us beyond the weak-field limit of the solar-system in the
study of theories of gravity. Their contribution is crucial as no test can be
considered to be complete without probing the strong-field realm of
gravitational physics by finding and timing pulsars. This is particularly
highlighted by the discovery of the first double pulsar system in 2003. In this
review, I will explain some of the most important applications of millisecond
pulsar clocks in the study of gravity and fundamental constants.
