The widespread applications of thermoelectric (TE) materials in power generation and solid-state cooling require improving their TE figure of merit (ZT) significantly.Recently,GeTe-based alloys have shown great promise as mid-temperature TE materials with superhigh TE performance,mostly due to their relatively high-degeneracy band structures and low lattice thermal conductivity.In this perspective,we review the most recent progress of the GeTe-based TE alloys from the view of phase and defect engineering.These two strategies are the most widely-used and efficient approaches in GeTe-based alloys to optimize the transport properties of electrons and phonons for high ZT.The phase transition from rhombohedral to cubic structure is believed to improve the band convergence of GeTe-based alloys for higher electrical performance.Typical defects in GeTe-based alloys include the point defects from Ge vacancies and substitutional dopants,linear and planar defects from Ge vacancies.The defect engineering of GeTe-based alloys is important not only for optimizing the carrier density but also for tuning the band structure and phonon-scattering processes.The summarized strategies in this review can also be used as a reference for guiding the further development of GeTe-based alloys and also other TE materials.