Analytic solutions to the Einstein field equations for a point particle possessing mass, angular momentum, and electric charge are widely assumed to provide an exact representation for astrophysical 'black holes.' These objects are inferred from measurements of the electromagnetic spectrum of associated accretion disks and kinematic observations of nearby luminous objects which indicate large amounts of mass contained within a few Schwarzschild radii.
Such objects are unique for the assumed simplicity with which they can be completely parametrized given the scale of the entropy they possess, and are impossible to observe directly. The implication, therefore, is that one must be careful in relying on the canonical model of these objects, particularly in light of the possibility of general relativity being supplanted by an alternative theory of gravity.
The observational evidence for compact objects distinct from more traditional dense matter is reviewed as context, and tests of the Kerr metric as a model for these objects are discussed.