Flaviviruses have traditionally been considered to be transmitted by blood sucking arthropods, such as mosquitoes or ticks, to susceptible vertebrate hosts. They replicate in both the arthropod vector and the vertebrate host, and include important human pathogens such as dengue, West Nile, and Zika viruses. In recent times we have also discovered a number of novel flaviviruses (referred to as insect-specific flaviviruses - ISFs) that replicate only in mosquito cells and show no or very limited replication in vertebrate cell cultures. Between 2010 and 2016 we identified at least 8 new flaviviruses from Culex, Coquillettidia, Aedes and Anopheles mosquitoes that exhibit an ISF-like phenotype. Our evidence to date suggests that, in the absence of vertebrate hosts, most of these viruses are transmitted directly from the female mosquito to their progeny via the infected egg. When we further examined the ability of these viruses to infect and replicate in cells from a range of mosquito species, we observed a considerable variation in their mosquito host range. Aedes-associated ISFs replicated in cells from Ae. vigilax, Ae. albopictus and Ae. aegypti but not in cells derived from Culex or Anopheles species. A novel clade of Anopheles-associated ISFs that we have recently identified, were shown to be highly specific to the mosquito species in which they were detected, suggesting extensive adaption to their host species. At the other end of the spectrum, ISF-like viruses from Coquillettidia and Culex species replicated in all mosquito cell lines tested. One flavivirus obtained from Cx. annulirostris, which failed to grow in several vertebrate cell lines used to culture “traditional” flaviviruses, only replicated efficiently in a marsupial cell line derived from the opossum (Didelphis sp.). These data provide a unique perspective on the host range of flaviviruses and new insights into their evolution and ecology. The new viruses we have identified will also be useful models to study the viral and host mechanisms involved in flavivirus host restriction.