Reproducing sharks must provide their offspring with an adequate supply of nutrients to complete embryonic development. In oviparous (egg-laying) sharks, offspring develop outside the mother, and all the nutrients required for embryonic growth are contained in the egg yolk. Conversely, in viviparous (live-bearing) sharks, embryonic development is completed inside the mother, providing offspring with the opportunity to receive supplementary embryonic nourishment, known as matrotrophy. Viviparous sharks exhibit nearly all forms of matrotrophy known in vertebrates, including a yolk-sac placenta, which involves several significant ontogenetic modifications to fetal and maternal tissues. The selective pressures that have driven the evolution of complex placentas in some shark species, but not in others, are unresolved. Herein we review the mechanisms of reproductive allocation and placental diversity in sharks, and consider the application of both adaptive and conflict hypotheses for the evolution of placental nutrient provisioning. Both have likely played roles in placental evolution in sharks, perhaps at different times in evolutionary history. Finally, we recommend sharks as an outstanding model system to investigate the evolution of placentas and mechanisms for fetal nutrition during pregnancy in vertebrates. Live-bearing reproduction provides developing embryos with the opportunity to receive maternal nutrients during pregnancy, in addition to the yolk. In some sharks, this transfer of nutrients is achieved by a placenta. Herein we consider the adaptive significance and evolution of the placenta in sharks. We recommend sharks as important models to investigate the evolution of vertebrate pregnancy and modes of fetal nutrition. [ABSTRACT FROM AUTHOR]