Arboviruses, including dengue virus (DENV), yellow fever virus (YFV), and Zika virus (ZIKV), pose a significant health and economic threat worldwide. These orthoflaviviruses are transmitted by Aedes aegypti mosquitoes, which are expanding their distribution to increase the global burden of disease. Therefore, understanding how these virus-host interactions are crucial. Our understanding of mosquito immunity largely stems from fruit fly models, which can exhibit varied responses to identical viruses. The mosquito Imd pathway is a NF-κB immune signalling pathway, that has been best characterised for its role in antibacterial and antifungal defence. This thesis sought to address key knowledge gaps regarding the overall antiviral effects of this pathway at a cellular level as well as the mechanisms by which viruses are sensed by and antagonise the pathway. YFV and ZIKV were shown to replicate earlier and to higher levels in A. aegypti cells deficient in NF-κB signalling, however, knocking out NFκB signalling does not allow all orthoflaviviruses to replicate. This indicates that NF-κB signalling is an important antiviral defense, but not the only barrier influencing the species tropism of orthoflaviviruses. Mosquito-borne orthoflaviviruses were subsequently shown to have the conserved ability to antagonise the Imd pathway response during viral infection, a mechanism for which the nonstructural protein, NS4A was shown to be partly responsible for. This mechanism of innate immune antagonism was found to be vector-specific, as only NS4A from Aedes-borne orthoflaviviruses antagonised the Imd pathway in A. aegypti cells. Further work to predict the structure of NS4A was carried out by the machine learning tool Alphafold2, with the goal of interrogating structure-function relationships. However, the predicted structures demonstrated numerous inconsistencies with functional data identified in the literature, reaffirming that Alpahfold2 still has flaws in accurately predicting transmembrane proteins and proteins that undergo conformational changes. Finally, attempts were made to characterise viral molecular patterns that stimulate the Imd pathway, and preliminary findings suggest that capped structured dsRNAs may be one of the molecular patterns sensed by the Imd pathway. This study reveals that the NF-κB pathway in A. aegypti cells reduce the replication of Aedes-borne orthoflaviviruses, and the NS4A protein encoded by these viruses, can antagonise the Imd pathway, which is potentially induced by the sensing of capped dsRNA structures. In the future, this information could generation of A. aegypti mosquitoes that cannot contribute to orthoflavivirus transmission.