Plastic deformation of yttria tetragonal zirconia polycrystals (YTZP) has been the objective of utmost interest owing to the exceptional properties such as superplasticity exhibited for grain sizes in the submicrometer range. In theory, this property is expected to improve with decrease in the average grain size. Therefore, the study of nanometer-sized YTZP ceramics is receiving an increasing attention in the ceramic scientific community. This paper is a critical analysis of the role played by yttrium segregation at the grain boundaries as a driving force to account for the peculiarities of the mechanical response of these ceramics.