The ntop project was started ten years ago with the goal of developing an open-source application for passive traffic monitoring. Over time ntop has been adapted to WAN traffic monitoring and today it can be effectively used for passive packet capture and analysis, as well as collector for popular protocols such as NetFlow/IPFIX and sFlow. The talk will discuss the challenges of passive traffic monitoring and will highlight the various open-source tools developed by the author in the past ten years for facing with such challenges.