The history, breeding biology, and structure of the Caspian Tern population of the Great Lakes from 1896-1964 was analyzed on the basis of band recoveries of chicks tagged 1922-1953, and from data gathered in a survey of the colonies from 1959-1964. Caspian colonies were found on islands with sand-gravel or limestone substrate and little vegetation. An estimated 1,620 breeding pairs were present in 1964. Mean clutch size was 2.81, Hatching rate .81, and Fledging Rate 1.61 chicks per pair of adults in 1963 and 1964. Juveniles disperse rapidly from the colonies in August, wintering on the Gulf Coast, Columbia, and Caribbean islands. Immatures are rather sedentary, spending their first full year on the wintering grounds. Adults summer on the lakes and winter in the same places as other age classes. The bulk of recoveries for all age classes come in the summer. The fewest recoveries are made in March and April. Over one-fourth of the Caspians recovered from the Great Lakes’ population are killed by shooting. The Great Lakes’ population is self-supporting. After declining slowly for 35 years, it reversed the trend, beginning to increase in 1960. Adults average 8.88 years of reproductive productivity, dying at a rate of 11.3 per cent per year. Increased fledging rate is cited as the immediate factor involved in changing the population structure. Preadult mortality was found to be 62% of the fledged young. This Caspian population has a structure similar to gull populations. Increasing food supplies in the form of Alewife are cited as the major reason for increasing tern numbers. Three-fourths of the fish eaten by Caspians while on the lakes in 1963 and 1964 were Alewife. No Lake Herring were found in the tern’s diet. The disappearance of Dibothriocephalus oblongatum from the Great Lakes’ population of the Caspian Tern is cited as further evidence that the ecology of the lakes has changed.