  The Square-Kilometre-Array (SKA) will be a radio telescope with a collecting
area that will exceed that of existing telescopes by a factor of a hundred or
so. This contribution summarises one of the key-science projects selected for
the SKA.
  The sensitivity of the SKA allows us to perform a Galactic Census of pulsars
which will discover a large fraction of active pulsars beamed to us, including
the long-sought for pulsar-black hole systems. These systems are unique in
their capability to probe the ultra-strong field limit of relativistic gravity.
By using pulsar timing we can determine the properties of stellar and massive
black holes, thereby testing the Cosmic Censorship Conjecture and the No-Hair
theorem. The large number of millisecond pulsars discovered with the SKA will
also provide us with a dense array of precision clocks on the sky. These clocks
will act as the multiple arms of a huge gravitational wave detector, which can
be used to detect and measure the stochastic cosmological gravitational wave
background that is expected from a number of sources.
