The rapid growth of the 2D MXenes family is driven by the designer chemistry control of their composition and structures, including the transition metal and surface functional groups, non-metal X sublattice and atomic-layer configurations. This compositional diversity controls the chemical ordering, atomic-level defects and surface chemistry, ultimately shaping properties of the MXenes. In this Review, we discuss how variations in compositional diversity and atomic arrangement give rise to material properties that enable new applications and breakthroughs in technology. We review design strategies, including atomic vacancy control, intercalation engineering and surface functionalization, that fine-tune the composition–property relationships in MXenes. In addition, we present emerging areas of MXenes research, including advances in biomedicine, optoelectronics, environmental remediation and catalysis, communication and quantum technologies, space exploration and thermal management.