We investigate whether black holes can be defined without using event
horizons. In particular we focus on the thermodynamic properties of event
horizons and the alternative, locally defined horizons. We discuss the
assumptions and limitations of the proofs of the zeroth, first and second laws
of black hole mechanics for both event horizons and trapping horizons. This
leads to the possibility that black holes may be more usefully defined in terms
of trapping horizons. We also show how Hawking radiation can also be seen to
arise from trapping horizons and discuss which horizon area should be
associated with the gravitational entropy.Comment: Invited review article for General Relativity and Gravitation. 43
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