  The study of the internal structure of star clusters provides important clues
concerning their formation mechanism and dynamical evolution. There are both
observational and numerical evidences indicating that open clusters evolve from
an initial clumpy structure, presumably a direct consequence of the formation
in a fractal medium, toward a centrally condensed state. This simple picture
has, however, several drawbacks. There can be very young clusters exhibiting
radial patterns maybe reflecting the early effect of gravity on primordial gas.
There can be also very evolved cluster showing fractal patterns that either
have survived through time or have been generated subsequently by some
(unknown) mechanism. Additionally, the fractal structure of some open clusters
is much clumpier than the average structure of the interstellar medium in the
Milky Way, although in principle a very similar structure should be expected.
Here we summarize and discuss observational and numerical results concerning
this subject.
