This lecture introduces the Macroscopic Fundamental Diagram (MFD) in traffic flow modeling, covering its existence, physical properties, and dynamic modeling. The instructor explains the need for a network-level approach due to the complexity of human choices and driving behavior. The lecture delves into the three regimes of the Fundamental Diagram for a link, the theory's generalization to networks, and simulation evidence supporting the MFD's existence. Real-world experiments and data fusion techniques using taxi and detector data are also discussed, highlighting the MFD's independence of Origin-Destination tables.