Pair-forming, courtship, mating, and post-copulatory behaviors are described for populations of Calopteryx maculata in the central and northeastern United States. Two pair forming displays were noted: (1) a display of the male's wings and abdomen (the cross display) at an oviposition site in his territory to an approaching female; and (2) a hovering courtship flight before a perching female. Similar displays are performed by C. aequabilis, C. splendens, and C. virgo, although intra- and interspecific variations occur in the cross display and its position in the pair-forming sequence. Both displays present the species and sex-specific color patterns of the male's wings and abdomen to the female early in pair formation. Pair formation differs in the three types of male-female encounters observed: (1) female flies into the male's territory; (2) male discovers an ovipositing female; and (3) male discovers a female on shore. The first type involves only territorial males and is the only one in which the cross display is performed. The other encounters involve both territorial and nonterritorial males. Courtship flight is present in the pair-forming sequences of all three encounter types. The cross display identifies a particular male with an oviposition site in his territory. This makes it possible for females to choose among males on the basis of the suitability of these oviposition sites. Females mating with territorial males benefit from reduced interference from other males during oviposition. The selective advantages of territoriality for a Calopteryx male are: (1) an increase in his mating frequency by the exclusive occupation of an oviposition area attractive to females ; and (2) an increase in the number of eggs he fertilizes by protecting mated females during their oviposition. It is likely that territorial behavior in Calopteryx functions primarily in obtaining and selecting mates and secondarily in reducing interference with pair formation and oviposition. Sexual selection among males for attracting mates and lessening sperm competition may have been an important factor in the evolution of territorial behavior in Calopteryx.