"Binary and triple systems of supermassive black holes in galactic nuclei": Galaxies with stellar bulges are generically observed to host supermassive black holes (SMBHs), and the hierarchical merging of galaxies therefore leads to the formation of SMBH binaries. In the low-density, gas-poor nuclei of giant elliptical galaxies, these binaries are expected to survive over times much longer than a Hubble time, leading to the formation of triple SMBH systems. I will discuss the dynamics of binary and triple SMBH systems in the stellar environments typical of elliptical galaxies, as well as the signatures of these systems in forthcoming pulsar timing experiments designed to detect low-frequency gravitational waves. I will highlight several open problems related to SMBH binary evolution in galactic nuclei, including the eccentricity evolution of strongly bound binaries and the effect of triaxiality of the stellar distribution on the SMBH evolution.