SummaryIt is demonstrated that third-harmonic generation (THG) near interfaces in the refractive index or the third-order nonlinear susceptibility (x (3) ) permits three-dimensional imaging of transparent objects. The nonlinear dependence of THG on the excitation power provides inherent optical sectioning. At the same time, the nonresonant nature of THG, in combination with the near-IR excitation wavelengths used (1-2 mm), render this technique potentially (biologically) nondamaging and nonbleaching. A specific property of THG imaging is its sensitivity to -and potential use for imaging of -the relative orientation of interfaces with respect to the axis of propagation of the excitation radiation.