 In this lecture, we have given an high-level introduction to Internet measurement, in particular to network flows. We have first talked about what, why, and how to measure the Internet. We have looked into active measurements, giving some telling examples of measurements, and our existing measurement platform. We have then looked at passive measurement and network flows in particular, reasoning on the abstraction problem introduced by measuring at Corsa granularities. We then have proposed you an in-depth presentation of network flows. In this set of slides, we have seen an entire flow monitoring architecture in detail, from packet observation to data analysis application. We then have seen and reasoned about possible measurement artifacts that are introduced by the flow creation process, and that we, as developers of flow-based analysis applications, should be aware of.