An overview of recent research on vibronic transitions of intraconfigu
rational 4f(n) transitions in rare earth ions is given. The strong var
iation in vibronic coupling strengths is shown to depend on the type o
f rare earth ion and the chemical nature of the ligands. In the discus
sion on the mechanism of vibronic coupling, new evidence for the impor
tance of DELTA process vibronics (or Franck-Condon phonon replicas) ne
xt to the usually considered M process vibronics (or vibrationally ind
uced forced electric dipole transitions) is presented. Concentration e
nhancement of vibronic transitions in the excitation spectra of Pr3+ i
n several covalent host lattices is reported. A model based on superex
change interaction between Pr3+ ions over distances of about 11 angstr
om can account for the observed concentration enhancement.