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Transitions between water and land have occurred multiple times in vertebrate evolutionary history. Secondary land-to-water transitions are often accompanied by characteristic evolutionary changes in morphology, including a shift from tubular limbs to flattened flippers. Differences in limb structure across taxa are often attributed to differences in skeletal loading. However, empirical data on loading differences between land and water are lacking, making it difficult to evaluate which mechanistic changes accompany morphological adaptations in lineages that shift from terrestrial to aquatic habitats. I used turtles as a model lineage for examining structural and functional implications of differences in limb bone loading between water and land. My examination is comprised of four studies. First, I compared loading regimes for the femur of semi-aquatic sliders (Trachemys scripta) during walking and swimming. These trials generated empirical data to test assumed loading differences between water and land. As the extent of limb flattening in many secondarily aquatic tetrapods is especially pronounced in the forelimb, compared to the hindlimb, I next compared loading of the humerus during walking and swimming in the semi-aquatic river cooter (Pseudemys concinna). Turtles have transitioned between land and water several times throughout evolutionary history, and such historical transitions may have influenced morphological adaptations of extant taxa. To examine this potential, I compared the swimming kinematics of four turtle species that included two semi-aquatic taxa (Chrysemys picta and T. scripta) and two independently evolved terrestrial specialists (Testudo horsfieldii and Terrapene carolina). This work evaluated the retention of ancestral swimming ability among taxa that have shifted to terrestrial habitats. Finally, it is difficult to assess how differences in loading between land and water may have influenced the morphological diversity of turtle limbs without considering data from taxa that span a complete range of locomotor habits. I collected morphological data from four functionally divergent clades, and calculated allometric comparisons of humerus and femur shape using phylogenetic comparative methods to test for divergence in limb bone morphology among taxa that use different habitats. Together, these studies provide biomechanical, kinematic, and phylogenetic insight to the mechanisms influencing the evolution of limb morphology associated with secondary aquatic invasions.