AbstractFoundations of thermodynamics are reviewed in terms of the elementary processes driving spontaneous evolution in physical systems. These processes lead to ergodic and mixing behavior as well as to conservation of macroscopic quantities such as total energy; finally a tendency towards order through instabilities results when the environment is cold enough. Instabilities are in turn considered as processes driving a spontaneous evolution in complex systems in their natural environment; again ergodic and mixing behavior is exhibited as well as new conserved quantities. As a result, a second thermodynamics is built which relates complexity to the corpuscular nature of matter. Thus, general properties of complex systems are pointed out, such as spontaneous and irreversible tendency towards complexity, development by stages and information flow between systems of different complexity.