"Psychologie des foules" by Gustave Le Bon is a psychological study published in 1895. This influential work examines how individuals behave when absorbed into crowds, arguing they lose rational thought and become impulsive, emotional, and susceptible to manipulation. Le Bon explores crowd characteristics, the role of leaders and persuasion, and analyzes different crowd types from criminal mobs to electoral assemblies. The book profoundly influenced later thinkers including Freud and shaped twentieth-century understanding of mass psychology and collective behavior.