Since the first report of ferroelectricity in a Si-doped HfO ₂ film in 2011, HfO ₂-based materials have attracted much interest from the ferroelectric materials and devices community. However, in HfO ₂-based bulk materials, the ferroelectric phase is not the one with the lowest free energy. It is, therefore, crucial to identify the possible thermodynamic and kinetic drivers for such an unexpected phase formation. The main difference between this type of material and conventional perovskite-based ferroelectrics is the movement of oxygen ions upon polarization switching, which complicates the structural examination of samples. Nonetheless, concerted efforts in academia and industry have substantially improved our understanding of the material properties and root causes of the unexpected formation of the ferroelectric phase. These insights help us understand how to induce the polar phase even in bulk materials. In this Review, we discuss in depth the properties and origin of ferroelectricity in HfO ₂-based materials, carefully evaluating numerous reports in the field, which are sometimes contradictory, and showing how thermodynamic and kinetic factors influence phase formation almost equally. We also survey possible applications and prospects for further development.