 Welcome back to this course. The topic of this week will be network measurements. Our first question is why do we need to measure the Internet? Researchers have identified that the reason to measure the Internet can be roughly classified in commercial, social and technical. In truth, most of the topics that nowadays are investigating by using network measurements span over several of those areas. Internet measurements are a support methodology for a large spectrum of problems. Think, for example, to how to manage and diagnose a network or plan a future network extension or troubleshooting a network. Also, the rise of social networks, mobility and the use of the Internet as supporting infrastructure for activism to cite some factors have prompted study that aimed at quantifying and characterizing the human factor. In this course, we are interested in Internet measurement as a means to identify, characterize and understand network security. This week, we will investigate the following topics. After this introduction, we will provide an overview of Internet measurements. Then, we will look in depth into the concept of network flows and how flows are monitored and measured. Finally, we will conclude.