A brief survey on the Virtual Observatory

This paper presents a short survey on the astronomical Virtual Observatories (VOs) that are part of the International Virtual Observatory Alliance (IVOA). From how they are distributed worldwide to their specialization range on the electromagnetic spectrum, we summarize key aspects of the current 21 VO members. Through the registry service, we explore the resources that the VO offer as a whole, and identify that even though the VO is already a mature initiative, there are important challenges to address in the near future.