χSecond and third harmonic generation of far-infrared radiation in bulk solids is reported The far-infrared source is a step-tunable gas laser emitting in the range 8.2 to 57 cm-1. Single pulses with power of 100 KW and duration of 40 ns are focused on dielectrics or semi-conductors. Harmonic power is selectively transmitted through perforated metal cutoff filters and detected with a GaAs photoconductor. Harmonic powers up to 100 W are obtained. Quantitve measurementllow to determine absolute values of the nonlinear susceptibilities: )0 /(2w,w,w) and )0 1(3w,w,w,w). In noncentrosymmetric infraredactive crystals )0(2 ) exhibits a lattice contribution which comes in addition to the electronic contribution usually dealt with in near-infrared and visible nonlinear optics. The lattice contcligution is resonant near the optical phonon or Reststrahlen frequency. We have measured )0 ion the low-frequency wing of the lattice resonance of LiTa03 and found it consistent with theory. Free cmriers in semiconductors exhibit a surprisingly large third-order nonlinear susceptibility x iprovided the frequency is low enough. Typically x iincreases by 7 orders of mmitude when the wavelength increases from 10 μm to 500 μm. We have absoluly determined )0 lby measuring the efficiency of frequency tripling at 20 cm-1. We found xk /proporti9941 to the density n of free electrons or holes for n < 1017 cm-3. The absolute value is )0 " 10-4 e.s.u. for n = 1017 cm-3 in a variety of semiconductors. The theoretical analysis allows to distinguish between two contributions, (i) due to the nonparabolicity of the carrier dispersion and (ii) due to the energy dependence of carrier scattering. Altogether the free carrier nonlinearity offers applications of efficient four-wave mixing employing at least one high-intensity far-infrared beam.