Skype is a software application that allows users to make telephone calls over the Internet. Calls to other users of the service, and in some countries to free-of-charge numbers are free, while calls to other landlines and mobile phones can be made for a fee. Additional features include instant messaging, file transfer and video conferencing.

Skype was written by Estonia-based developers Ahti Heinla, Priit Kasesalu and Jaan Tallinn, who had also originally developed Kazaa. The Skype Group, founded by Swedish-born entrepreneur Niklas Zennström and the Dane Janus Friis, has its headquarters in Luxembourg, with offices in London, Tallinn, Tartu, Stockholm, Prague, and San Jose.