Abstract: Self-organized behaviors are commonly observed in nature and human societies, such as bird flocks, fish swarms and human crowds. In this talk, I will present some celebrated mathematical models, with simple small-scale interactions that lead to the emergence of global behaviors: aggregation and flocking. In particular, I will focus on the Euler-alignment system, and present some progress on the global wellposeness theory, large time behaviors, as well as interesting connections to classical equations in fluid mechanics.