One of the primary goals of biology is to explain patterns of phenotypic diversity. One trait in animals with considerable diversity within and between species is brain form (i.e. brain size and the proportional relationships between brain parts). Variation in brain form affects cognitive ability through variation in neuroprocessing power. Ecological conditions are hypothesized to generate different cognitive requirements, potentially driving the evolution of brain form. I study the link between ecological conditions and brain from using fish because fish are extremely diverse in terms of their ecology and their brain form. In my thesis, I answer three primary questions. First, why may fish brain form vary independent of body size? Second, what factors may constrain variation in brain form? Finally, what proximate mechanisms contribute to variation in brain form? I test for the effect of ecology on brain form variation using populations of sunfish that are diverging between the littoral and pelagic habitats of lakes. These habitats differ in their biotic and abiotic characteristics, potentially leading to differences in the level of cognitive ability required for optimal ecological performance. I address these research questions using interspecific and intraspecific comparisons, ontogenetic comparisons using field samples, and with a lab-based rearing experiment designed to evaluate heritable variation and plastic responses in brain form. The ecological divergence of sunfish between lake habitats influences variation in brain form in all comparisons, consistent with the hypothesis that habitats vary in their cognitive requirements. However, inconsistencies in the specific patterns of brain form divergence between habitats suggests that constraints exist that limit the evolution or development of optimal brain form. Additionally, I found a mismatch between brain size development and ontogenetic shifts in ecology, again consistent with an important constraint on brain growth over development. Finally, experimental results indicate that divergence in brain size between habitats reflects evolved differences in brain size plasticity between ecotypes. Broadly, my thesis demonstrates the importance of ecological variation in shaping brain form evolution within a set of limitations that are only beginning to be revealed, and so underscores the challenges to understanding the evolution and ecology of brain form.