Transition-metal selenides (MxSey, M=Fe, Co, Ni) and their composites exhibit good storage capacities for sodium and lith-ium ions and occupy a unique position in research on sodium-ion and lithium-ion batteries. MxSey and their composites are used as active materials to improve catalytic activity. However, low electrical con-ductivity, poor cycle stability, and low rate performance severely limit their applications. This review provides a comprehensive introduction to and understanding of the current research progress of MxSey and their composites. Moreover, this review proposes a broader research platform for these materials, including various bioelectrocatalytic per-formance tests, lithium–sulfur batteries, and fuel cells. The synthesis method and related mechanisms of MxSey and their composites are reviewed, and the effects of material morphologies on their electro-chemical performance are discussed. The advantages and disadvan-tages of MxSey and their composites as well as possible strategies for improving the storage and conversion of electrochemical energy are also summarized.