The term "complexity" is currently used in the study of non-linear dynamics. The main problem with this term arises when is applied to biological systems. For example, does evolution generate ever more complex organisms? A good measure of complexity is lacking. We can observe complexity at structural, functional or "grammatical" levels, seeing the number of specialized cells in one organism, its functions and the nesting of functions, or some relationship between genome sequence and structure, function or, more generally, fitness to environment. The question remains: How could we measure complexity in biological systems?