<p>Zinc oxide (ZnO) is a technologically important wide-band-gap oxide whose functional performance is strongly governed by surface defects. Its polar crystal structure and rich defect chemistry give rise to complex surface reconstructions, defect states, and adsorbate interactions that critically influence electronic, chemical, and catalytic behavior. This review provides a comprehensive and critical assessment of ZnO surface defects, integrating insights from scanning probe microscopy, photoelectron spectroscopy, and density functional theory calculations. The nature, formation, and stability of intrinsic defects—including oxygen vacancies, zinc-related defects, and defect complexes—are discussed in relation to surface polarity compensation, electronic structure modification, and band bending. Reported oxygen vacancy formation energies (~ 2.5–5.0&#xa0;eV) and O 1s XPS features (~ 530&#xa0;eV for lattice oxygen and ~ 531–532&#xa0;eV for hydroxyl species or adsorbates) highlight persistent challenges in defect identification and interpretation. The implications of surface defects for functional applications, including photocatalysis, CO₂ photoreduction (with product yields ranging from μmol to mmol.g⁻¹&#xa0;h⁻¹), gas sensing, and optoelectronic performance, are critically evaluated. Emphasis is placed on the dominant role of surface chemistry and defect–adsorbate interactions. This review outlines current challenges and future strategies for rational defect engineering of ZnO surfaces toward improved material performance.</p> Graphical abstract <p></p>