This chapter discusses the characteristics, taxonomy, distribution, abundance, and ecology of the leopard seal or Hydrurga leptonyx. Leopard seals are a large sexually dimorphic species. The females are larger than the males, growing up to 3.8 m in length and weighing up to 500 kg, whereas males grow up to 3.3 m in length and weigh up to 300 kg. The leopard seal population is estimated to be 222,000-440,000. During summer, leopard seals breed on the outer fringes of the pack ice where they are solitary and sparsely distributed. Their density is inversely related to the amount of pack ice available to the seals as haul-out platforms. Pack ice cover varies with the season, from a maximum between August and October to a minimum between February and March. Population densities are greatest in areas of abundant cake ice and brash ice, whereas they are least in areas with larger floes. Leopard seals take a diverse range of prey including fish, cephalopods, sea birds, and seals. Different food sources are used when available or when opportunities to take other more sought-after prey are few. Krill makes up the largest proportion of their diet, particularly during the winter months when other food types are not abundant.