We discuss, in terms of particle acceleration, the results from optical observations with ESO's Very Large Telescope aimed at the detection of the optical counterparts of radio hot spots. To select the targets, we adopted criteria based on theoretical expectations and obtained an unprecedented 70% detection rate. Our results strongly suggest that the optically emitting relativistic electrons are accelerated in situ in the hot spot regions. In particular, the break frequencies of the synchrotron spectra increase with decreasing magnetic field strength in the hot spots. In addition, the presence of kpc scale diffuse optical emission associated with the hot spots suggests the presence of additional acceleration mechanisms (possibly Fermi II-like) working in the post shock region.