Combining two conventional materials with distinct coefficients of thermal expansion at the microscale enables fabrication of metamaterials with negative, zero, or custom-designed thermal expansion coefficients, thereby mitigating harmful structural deformations and thermal mismatches in high-end industrial equipments. As a major branch of mechanical metamaterials research, these structures are evolving rapidly toward multifunctionality. Their chief advantage lies in high designability (tunability), which permits on-demand property customization. Fundamental realization principles and control mechanisms of tunable thermal-expansion metamaterials are elucidated, followed by a systematic overview of advances in mechanical reinforcement, integration of unconventional properties, dynamic performance regulation, and controllable thermal-expansion superstructures. Current design methodologies, especially the pivotal role of topology optimization in innovative configuration development, are then examined. The selected aerospace applications highlight practical potential. Key research challenges are identified, and future development trends are proposed.