Abstract
Entropy is fundamental to describing open systems. It makes possible the distinction between non-equilibrium and equilibrium processes. The influx of negentropy is a measure of all physical and chemical processes occurring in a system. The entropy balance equation is a useful tool for the comprehensive description of an open system, as its application to planets and ecosystems illustrates. The entropy-information relationship established by the theory of information is applicable to self-organising systems provided their special features, particularly the presence or absence of memory, are taken into account.