In short, there are no ads.
Due to the fact that Viber is completely free both as an application and service, many people are asking will Viber in the future add advertisements to its current services. Well, here is the official statement regarding this issue:
1. The Viber software will always be free.
2. Viber will never display any ads.
3. Viber to Viber calls will always be free.
4. Viber to Viber text messages will always be free.