Pyrethroid insecticides are commonly used to control Aedes aegypti, an important human disease vector; however, insecticide resistance is spreading rapidly amongst this mosquito species. Interactions between the two major mechanisms of pyrethroid resistance, cytochrome-P450-monooxygenases (CYPs)-mediated detoxification and voltage-sensitive sodium channel (Vssc) target gene modifications (known as kdr), can influence the rate that pyrethroid resistance evolves and therefore, influence the level of resistance in a population. Little is known about the evolution of the individual resistance mechanisms. Few studies have looked at how the interaction between the mechanisms affect resistance and none have compared how this interaction affects fitness. My goal was to study this interaction in two ways: how the mechanisms interact both in the presence of insecticides (i.e. their levels of resistance) and in the absence of insecticides (i.e. their fitness costs). I used three congenic strains containing the different resistance mechanisms alone and in combination: CYP:ROCK (CR) that contains only CYP-mediated detoxification; KDR:ROCK (KR) that contains only kdr; and CYP+KDR:ROCK (CKR) that contains both kdr and CYP-mediated resistance. I then assessed the resistance and cross-resistance of several pyrethroids and found that resistance varied by insecticide. In general, kdr confers greater resistance than CYPs and in combination, the resistance effect is greater-than-additive. Measuring fitness can be challenging because different types of experiments can resolve fitness costs differently, and synthesis of these results can be difficult. Here, I evaluated the fitness costs based on both specific fitness parameters (i.e. life table and mating competition), and by measuring overall fitness via an allele-competition study. Our life table results show a strong cost in the strains containing CYP-mediated resistance (CR and CKR), but not for kdr (KR). CKR males had a reduced mating success relative to ROCK males when attempting to mate with ROCK females. The allele-competition study results showed a significant decline in the kdr allele both in the presence and absence of CYP-mediated resistance, indicating a high cost for this allele. I found no correlation between the level of insecticide resistance conferred by the different mechanisms and their fitness cost. Understanding fitness costs is important for resistance management practices because it helps determine whether and how quickly resistance will be lost after pesticide application has ceased.