In this article, we present a self-contained review of recent work on complex biological systems which exhibit
no characteristic scale. This property can manifest itself with fractals (spatial scale invariance), flicker noise
or 1/f-noise where f denotes the frequency of a signal (temporal scale invariance) and power laws (scale
invariance in the size and duration of events in the dynamics of the system). A hypothesis recently put
forward to explain these scale-free phenomomena is criticality, a notion introduced by physicists while
studying phase transitions in materials, where systems spontaneously arrange themselves in an unstable
manner similar, for instance, to a row of dominoes. Here, we review in a critical manner work which
investigates to what extent this idea can be generalized to biology. More precisely, we start with a brief
introduction to the concepts of absence of characteristic scale (power-law distributions, fractals and 1/f-
noise) and of critical phenomena. We then review typical mathematical models exhibiting such properties:
edge of chaos, cellular automata and self-organized critical models. These notions are then brought together
to see to what extent they can account for the scale invariance observed in ecology, evolution of species, type
III epidemics and some aspects of the central nervous system. This article also discusses how the notion of
scale invariance can give important insights into the workings of biological systems.
