Zika virus (ZIKV) is yet another arbovirus that is rapidly emerging on a global scale, on the heels of a chikungunya epidemic in the Americas that began in 2013. A ZIKV epidemic that began in Brazil in 2015 has now spread rapidly to more than 30 countries in the Americas and the Caribbean, infecting more than 2 million inhabitants. This epidemic currently continues unabated. The explosive nature of recent outbreaks and concerning links to Guillain-Barré syndrome and microcephaly are incompletely understood. Also unknown is the relative importance of sexual transmission of ZIKV and asymptomatic ZIKV infections to the overall burden of transmission. The limited understanding of ZIKV presents an enormous challenge for responses to this rapidly emerging threat to human health. This article reviews the existing literature on ZIKV and proposes critical questions for vaccine development and other areas of needed research.