Three pathogenic mosquito-borne viruses that have been on the rise in recent years are Zika virus (ZIKV), Usutu virus (USUV) and Mayaro virus (MAYV). ZIKV unexpectedly caused a large-scale epidemic of human illness in Central and South America in 2015 and 2016. The zoonotic USUV has recently spread throughout Europe, causing massive bird die-off and rare but severe neuroinvasive disease in humans. The tropical MAYV is currently emerging in Central and South America, and infection in humans can result in long-lasting, debilitating arthralgia. The rapid emergence of these three arthropod-borne (arbo)viruses urged for an in-depth analysis of the mosquito vectors capable of transmitting these viruses, as well as for the development of effective strategies to confine and prevent epidemics of these arboviral diseases.Although the number of ZIKV cases has declined after the outbreak in the Americas, the virus is still present in tropical regions and therefore remains a threat to public health. Especially in areas with human populations naive to the virus, ZIKV may suddenly emerge, which could result in new, major disease outbreaks. Since arboviruses replicate in both their vertebrate host and invertebrate vector, the risk of ZIKV outbreaks in a particular region is also determined by the presence of competent mosquito vectors. In this thesis, it was investigated how effectively indigenous and invasive mosquito species present in the Netherlands transmit ZIKV in the laboratory. The invasive Asian bush mosquito Aedes japonicus, permanently established in Flevoland, the Netherlands, was found capable of experimentally transmitting ZIKV, hence suggesting that this mosquito species could be a vector for ZIKV. The indigenous common house mosquito Culex pipiens, however, was unable to transmit ZIKV after an infectious blood meal. Nevertheless, bypassing the mosquito midgut by intrathoracic injection of ZIKV resulted in limited virus accumulation in Cx. pipiens saliva. This indicates that the mosquito midgut normally restricts ZIKV dissemination in Cx. pipiens after oral exposure. Additionally, a general replication deficiency of ZIKV in Culex mosquito cells was identified, which occurred post-entry. These results indicate that Cx. pipiens should be considered a highly inefficient vector for ZIKV.Cx. pipiens mosquitoes can, however, effectively transmit USUV. This virus is maintained in an enzootic transmission cycle between birds and mosquitoes. Humans and other mammals can also become infected via mosquito bites but are thought to be dead-end hosts due to low levels of viraemia. Thus, when local mosquitoes are evaluated for their ability to transmit USUV under experimental conditions, the use of avian blood for the infectious blood meal would be preferable. Nonetheless, the origin of blood used to study vector competence generally varies between studies, while it is unknown to what extent the blood source affects the experimental outcomes. In this thesis, it was found that the use of chicken or human blood resulted in comparable vector competence of Cx. pipiens for USUV. Interestingly, this study also revealed that the USUV titers in the saliva of the two biotypes of Cx. pipiens (pipiens and molestus) markedly differed. Biotype molestus accumulated much lower USUV titers in the saliva as compared to biotype pipiens, regardless of which blood type was offered. This may indicate that biotype molestus is a less efficient vector for USUV than biotype pipiens, which is especially interesting considering that biotype pipiens preferentially feeds on birds, whereas biotype molestus is more attracted to mammals including humans. Importantly, it was also found that the opportunistic feeder Ae. japonicus is capable of experimentally transmitting USUV, thus making this mosquito species a potential bridge vector between birds and humans.Besides arboviruses, mosquitoes can also carry insect-specific viruses (ISVs). Recently, ISVs have received increasing attention due to their ability to influence arbovirus transmission, and it is therefore important to characterize the collection of viruses (i.e., the virome) present in mosquito vectors. ISV replication in mosquito cells activates RNA interference (RNAi)-based immune responses, resulting in the production of virus-derived small RNAs. Sequencing and de novo assembly of these small RNAs provides an overview of the ISVs present in mosquito populations. In this thesis, novel virus species were discovered in Ae. japonicus populations in the Netherlands and France using a small RNA-based metagenomic approach. The newly discovered Ae. japonicus narnavirus 1 (AejapNV1) showed the strongest RNAi response. Narnaviruses have been described as positive-sense RNA viruses with only a forward open reading frame (ORF) coding for the RNA-dependent RNA polymerase (RdRp). Interestingly, AejapNV1 showed an ambigrammatic coding strategy with a forward ORF encoding the RdRp on the positive strand and a reverse ORF with unknown function on the negative strand. This was remarkable, as positive-sense RNA viruses usually code for proteins only on the positive strand.The arboviruses ZIKV and MAYV have the potential to invade new geographical areas, whilst no licenced antivirals or vaccines are available to treat or prevent disease. Here, virus-like particle (VLP) vaccines against both these viruses were developed using the scalable baculovirus-insect cell expression system. Vaccination of mice with MAYV VLPs induced high levels of neutralising antibodies, and completely protected the animals from viraemia and arthritic disease after challenge with wild-type MAYV, allowing this vaccine to be further developed for human use. Immunisation of mice with two developed ZIKV vaccine candidates, VLPs and subviral particles (SVPs), however, only induced limited levels of ZIKV-neutralising antibodies and did not protect against wild-type ZIKV infection, although the viraemic period became shorter. Epitope analysis showed that the ZIKV VLPs and SVPs do not display quaternary structure epitopes normally present on envelope protein homodimers found on the ZIKV virion. These epitopes induce potent neutralising antibodies following natural ZIKV infection in humans. To improve the efficacy of the ZIKV SVP vaccine, novel variants were developed with the specific aim to stabilise the envelope protein homodimers. The improved vaccine candidates now await further testing in mouse models of ZIKV disease.In conclusion, the results of this thesis enhance our understanding of the mosquito vectors involved in ZIKV and USUV transmission. Also, the developed VLP vaccines against ZIKV and MAYV will hopefully help to control outbreaks of these arboviral diseases in the future.