  The Interstellar Medium has a fractal structure, in the sense that gas and
dust distribute in a hierarchical and self-similar manner. Stars in new-born
cluster probably follow the same fractal patterns of their parent molecular
clouds. Moreover, it seems that older clusters tend to distribute their stars
with radial density profiles. Thus, it is expected that clusters form with an
initial fractal distribution of stars that eventually evolves toward centrally
concentrated distributions. Is this really the case? This simple picture on to
the origin and early evolution of star clusters and associations is very far
from being clearly understood. There can be both young clusters exhibiting
radial patterns and evolved clusters showing fractal structure. Additionally,
the fractal structure of some open clusters is very different from that of the
Interstellar Medium in the Milky Way. Here we summarize and discuss
observational and numerical evidences concerning this subject.
