In this lecture from the São Paulo School of Advanced Science on Disordered Systems, Professor Letícia Cugliandolo from Sorbonne Université, France, presents the second class on Complexity and Mean-field theory. Explore advanced concepts in disordered systems as part of the ICTP-SAIFR program scheduled for April 28 - May 9, 2025. The 93-minute presentation delves into theoretical frameworks for understanding complex systems through mean-field approaches, building upon foundations established in the first class. Gain insights into mathematical models that describe collective behavior in disordered materials and complex networks.