Ceramic lasers have advantages such as better optical homogeneity, cheaper production, and more freedom in shape compared with monocrystalline lasers. However, equal or better laser performances are required. Interface segregation is important as ceramics contain a high number of interfaces, segregation to which may locally alter the concentration of luminescent dopants, which in turn may influence laser performance. The present work applies atomistic simulation techniques to investigate the segregation of neodymium (Nd) dopants to surfaces and mirror twin grain boundaries in yttrium aluminum garnet (YAG). These results allow a better understanding of interfacial segregation and its influence on laser performance of Nd:YAG ceramics.