I thank the many researchers who answered my e-mail queries on various aspects of their work. I thank the anonymous referees for their careful reading of the manuscript and their many helpful comments. I thank Badri Krishnan for many useful conversations on the properties of apparent, isolated, and dynamical horizons. I thank Scott Caveny and Peter Diener for useful conversations on event-horizon finders. I thank Peter Diener, Luciano Rezzolla, and Virginia J. Vitzthum for helpful comments on various drafts of this paper. I thank Peter Diener and Edward Seidel for providing unpublished figures.
I thank the many authors named in this review for granting permission to reprint figures from their published work. I thank the American Astronomical Society, the American Physical Society, and IOP Publishing for granting permission to reprint figures published in their journals. The American Physical Society requires the following disclaimer regarding such reprinted material:
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I thank the Alexander von Humboldt Foundation, the AEI visitors program, and the AEI postdoctoral fellowship program for financial support.
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