The water vole is common in Middle and Late Pleistocene temperate Palaearctic faunas. It is widely used in biostratigraphy because of tem- poral trends in the size, shape and structure of the first lower molar (M1). However, geographic variation in the evolutionary development of the M1 has restricted the precision and accuracy of age-estimations. This thesis explores morphological variation in the M1 of fossil and extant populations of the lineage Mimomys savini–Arvicola, and uses the phenotype and the genotype to develop evolutionary hypotheses. Geometric and traditional morphometric methods are used to quantify tooth shape and enamel thickness from over 4000 digital photographs of M1s taken from specimens originating from 146 modern and fossil groups across the western Palaearctic. M1s are photographed to ob- tain a true cross-section, giving a more accurate description of molar shape. Morphological variation is explored in terms of sample-size, taphonomy, and ontogeny. Sample sizes of less than 10 are likely to provide inaccurate summary statistics of morphometric variables but depositional type appears to have no systematic effect on within-group variation. Change in the morphology of M1s through ontogeny is an im- portant source of morphological variation, explaining up to 29% of mo- lar shape within-specimens and up to 95% of enamel thickness within enamel layers of specimens. Removal of ontogenetic variation from mo- lar shape improves congruence between morphological and molecular data, indicating age-corrected variables should be used when assessing evolutionary patterns. Temporal and spatial patterns in the enamel thickness quotient (SDQ), based on age-corrected enamel thicknesses, mirror those from published data but differ in some details. Method- ological differences mean absolute SDQ values cannot be compared. Qualitative patterns include a large decrease in SDQ across MIS 12 and a steep east–west morphocline during the late Middle Pleistocene.