Alphaviruses are enveloped, positive-sense RNA viruses and include several agents that can cause lethal encephalitis when transmitted to humans through the bite of infected mosquitos. The cellular receptors these viruses attach to when infecting cells in mosquitos and humans, which are separated by hundreds of millions of years in evolutionary history, are in many cases unknown. I will describe how we used a CRISPR-Cas9 screen to identify the very low-density lipoprotein receptor (VLDLR) as an alphavirus receptor and additional data we obtained to show that orthologs from horses, birds, and mosquitos, which are relevant to the transmission cycle of alphaviruses, can also serve as functional receptors. Surprisingly, cryo-electron microscopy structures of VLDLR-bound Semliki Forest virus (an Old World alphavirus) and eastern equine encephalitis virus (a New World alphavirus) revealed that the spike proteins of both viruses use distinct surfaces to interact with VLDLR. I will also describe how additional structures of alphavirus spike proteins bound to cellular receptors in recent years have helped clarify the many distinct ways these viruses can attach to cellular receptors when infecting host cells.