Diatom identification must consider the large variability in both morphological and morphometric features, characteristic of this group of microorganisms. This chapter describes variations in shape/size observed in diatom populations either as a consequence of their particular asexual reproductive cycle or induced by environmental conditions. Concerning this latter, teratological diatoms are commonly associated with a variety of environmental stressors, particularly the presence of micropollutants in the aquatic ecosystem, such as heavy metals. We evidence an overestimation of water quality conditions caused by overriding deformed individuals in diatom-based biomonitoring studies. It can be shown that normal and aberrant forms of the same taxon differ in autecological preferences. Finally, we advise on a critical issue in the description of diatom specimens, that is, the sample size on which morphometric ranges should be provided. The section concludes with some recommendations in this regard.