The 20th century shows an essential change in young people's behaviour from Wandervogel, Boy Scouts and Komsomol to student rebellion, hippie, rock and pop, and techno cultures. These cultures show a new code of behaviour - a code of informality based on principles of symmetry, moratorium and modularity. The informal youth cultures develop as an attempt to respond to rapid social change and complexity by constructing an open order that can flexibly adjust to postmodern chaotic conditions. Based on empirical analyses of classical youth movements as harbingers of the code of informality, and of the recent example of Israeli youth movements, this study uses the above conceptual framework to explain the variety of youth behaviour in authentic rather than generational or conflictual terms. It sheds new light on youth movements and more recent expressions of youth in the same universe of informal youth structures. These informal structures institutionalize both youth authenticity and relation to adult society, constructing a context in which freedom and discipline coexist.