Supermassive black holes (SMBHs) may not always reside right at the centers of their host galaxies. This is a prediction of numerical relativity simulations, which imply that the newly formed single SMBH, after binary coalescence in a galaxy merger, can receive kick velocities up to several 1000？km/s due to anisotropic emission of gravitational waves. Long-lived oscillations of the SMBHs in galaxy cores, and in rare cases even SMBH ejections from their host galaxies, are the consequence. Observationally, accreting recoiling SMBHs would appear as quasars spatially and/or kinematically offset from their host galaxies. The presence of the “kicks” has a wide range of astrophysical implications which only now are beginning to be explored, including consequences for black hole and galaxy assembly at the epoch of structure formation, black hole feeding, and unified models of active galactic nuclei (AGN). Here, we review the observational signatures of recoiling SMBHs and the properties of the first candidates which have emerged, including follow-up studies of the candidate recoiling SMBH of SDSSJ092712.65+294344.0. 1. Introduction Interaction and merging of galaxies occurs frequently throughout the history of the universe. If both galaxies do harbor SMBHs, binaries will inevitably form [1]. Galaxy mergers are believed to be the sites of major black hole growth, and an active search for SMBH pairs and binaries of wide and small separations is currently ongoing (see [2] for a review of electromagnetic signatures). When the two SMBHs ultimately coalesce, they are a source of strong gravitational waves. These are emitted anisotropically during coalescence and carry away linear momentum (e.g., [3]). As a result, the newly formed single SMBH recoils. Configurations of coalescing black holes can lead to kick velocities up to several thousand km/s (e.g., [4–16]; review by [17]). In the initial computations, kick velocity was highest for maximally spinning equal-mass black hole binaries with antialigned spins in the orbital plane (“superkicks”). More recently, based on a new recoil formula, Lousto and Zlochower [18] have estimated that recoil velocities up to 5000？km/s can be reached in configurations with spins partially aligned with the orbital angular momentum. In unbound encounters (not likely to occur in astrophysical environments), the kick velocity can exceed 15？000？km/s [19, 20]. After the kick, the recoiling SMBH will oscillate about the core of its host galaxy [21, 22] or will even escape, if its kick velocity exceeds the escape velocity of its host. In a
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