This article presents a scientific overview of ‘entropy’ as a thermodynamic basic function. A primary definition of entropy is stated in classical thermodynamics and statistical mechanics. The second law of thermodynamics states that, in any physical or chemical process, the quality of the energy in a system degrades, thus proving the existence of irreversibility in nature. From the second law descends the existence of the extensive state function, namely ‘entropy’, which measures the disorder degree of a system, and it is also an evidence of the unidirectional flow of time. Besides this classical interpretation, the role of entropy and energy with respect to evolution of life and complex systems has been a subject of widespread interest among the scientific community in the past century. Taking into account the theories of Ilya Prigogine, the far-from-equilibrium systems theory and the pivotal concept of negentropy by Schrödinger, it is nowadays possible to use the concept of entropy as a powerful instrument to deal with ecosystems, ecology, and definitely life. The assessment of entropy production in ecosystems has been diffusely applied as a thermodynamic indicator for understanding the development state of ecological systems.