Abstract: The intricate arrangements of spins give rise to a wealth of magnetic phenomena, establishing magnetism as one of the most active and profound frontiers in condensed matter physics. Recently, the advent of altermagnets has opened a new avenue for exploring a third class of magnetic phases. This emergent phase is distinguished by the coexistence of spontaneously broken time-reversal symmetry, antiparallel magnetic order, alternating spin-splitting band structures, and symmetry-protected compensated magnetization. Integrating the advantages of both conventional ferromagnets and antiferromagnets, altermagnets enable a range of novel physical phenomena beyond traditional magnetic paradigms, including the lifted spin degeneracy, anomalous and spin transport effects, magnetooptical effects, and chiral magnons. These advances not only deepen our understanding of magnetism but also provide new theoretical frameworks and experimental opportunities for probing unconventional magnetic orders. Notably, theoretical predictions and experimental validations are advancing in parallel, with their synergy propelling altermagnetism from an emerging concept toward potential applications.