The electric-dipole intensities of Er3+ spectra in six different host lattices have been calculated on the basis of Judd-Ofelt theory, which takes into account the effect of a mixing of higher configurations of opposite parity into the fn configuration through odd parity spherical harmonics in the ligand field interactions. The magnetic-dipole intensities have also been calculated, and the sum of the contributions from these two mechanisms has been used to account for the observed intensities in the optical spectra of Er3+. Tje phenomenon of hypersensitivity is discussed. The complete energy matrices of the f11 electronic configuration have been diagonalized to calculate the energy levels of Er3+, and the eigenvectors thus obtained were used in the intensity calculations. The splittings of a few lowlying LS multiplets of the f3 configuration in the 'intermediate coupling' scheme are also shown schematically as a function of zeta /F2.