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The northern house mosquito, Culex pipiens L., is a vector of several diseases including West Nile virus, St. Louis encephalitis, and filariasis. Adult females of this species enter an overwintering dormancy known as diapause. Like most temperate insects, diapause in Cx. pipiens is programmed by the short daylengths of late summer and early fall although the molecular mechanism by which mosquitoes and other insects measure day length is unknown. Like other insects, Cx. pipiens, have circadian clocks which are composed of several genes whose mRNA transcripts and proteins oscillate throughout the day and, in doing so, regulate daily behaviors. In this thesis, I explore the possibility that circadian clock genes, which provide Cx. pipiens with information on the time of day, are also involved in measuring day length and hence initiating the adult overwintering diapause in this species.The clock genes in Cx. pipiens are highly similar to the circadian clock genes in the other mosquitoes and more distantly related insects. This suggests that the circadian clock in Cx. pipiens is similar in structure to other, non-drosophilid insects. To determine whether clock genes are differentially expressed during diapause, their expression was measured over a 24-hr period in nondiapausing mosquitoes as well as in females of Cx. pipiens throughout the diapause program and after diapause termination. I found that clock genes continue to cycle robustly throughout the diapause program, although early in diapause the expression of several core circadian clock genes show an altered expression profile with peak mRNA levels occurring later in the night than anticipated. This shift in clock gene expression may explain the shift in flight and feeding activity that has been previously observed in diapausing females of Cx. pipiens early in diapause. Using RNA interference (RNAi), the expression of several circadian clock genes was suppressed and the ability of females of Cx. pipiens to enter diapause was assessed. RNAi against negative circadian regulators, the genes period, timeless and cryptochrome2, caused females that were reared under short day, diapause-inducing conditions to avert diapause. In contrast, RNAi against the circadian-associated gene, pigment dispersing factor, caused females that were reared under long day, diapause-averting conditions to enter a diapause-like state. Together, these results suggest that clock genes control diapause initiation and perhaps diapause maintenance in Cx. pipiens.Additionally, the location of the circadian clock in the brains of Cx. pipiens was investigated by using immunohistochemistry with a primary antibody for the clock protein TIMELESS (TIM) derived from Drosophila melanogaster. TIM positive cells were found in the region between the optic lobes and the central brain, the subesophageal ganglion, and the pars intercerebralis (PI). Previous studies have demonstrated that the PI regulates the adult diapauses of other insects. Therefore TIM staining in the PI is the first step in establishing a physical connection between the circadian clock and the regions of the mosquito brains that control diapause in Cx. pipiens. Taken together, these experiments implicate circadian clock genes in the initiation and regulation of overwintering diapause in this important disease vector.