A system can be defined as complex if it is not equal to the sum of its parts. Even without defining the meaning of the concept “sum”, it is generally understood that most natural or man-made systems are complex. But complex is not the same as complicated. The latter can be defined and quantified, but complexity needs first a concrete qualitative characterization. There exist various approaches but the minimal ones are not widely applicable. Since complexity, whatever its definition is employed, is nearly always associated with uncertainty and probabilistic quantities, it is natural to consider quantities inherently related to information.