Field studies of the Horned Grebe (Podiceps auritus) were conducted during the breeding seasons of 1974 and 1975 near Minnedosa, Manitoba. This paper documents the reproductive biology of Horned Grebes and describes their adaptations for nesting in unstable marsh habitats. Nesting pairs defended Type A territories which ranged in size from 0.05 to 2.70 hectares. Occupation of a pothole by two or more nesting pairs occurred only on potholes larger than one hectare. Differential selection of permanent potholes for nesting reflected the grebes' dependence on open-water feeding areas. Interspecific territoriality by Horned Grebes was related to defense of their pairing and nesting platforms. Three factors were important in nest-site selections: (1) accessibility on the site from open water; (2) protection of the nest from wave action; and (3) concealment of the nest from predators. All nesting platforms were anchored to emergent vegetation. Annual variation in initiation of first clutches was correlated related with differences in air temperature during spring. Initiation of replacment clutches was influenced by the stage of the nesting cycle at which the previous nests were destroyed. Use of the same territory following failure of an incomplete clutch facilitated rapid clutch replacment. A seasonal decline in clutch size was observed and was possibly due to depleted energy reserves of females late in the nesting season and to an allocation of energy to the prebasic molt... The reproductive strategy of Horned Grebes is examined in relation to environmental stability, and possible selection pressures influencing this strategy are discussed.