Tightly focused beams of ultrashort optical pulses such as produced with a high numerical aperture optic as found in a microscope exhibit an interesting property: a strong third harmonic signal is produced when the axial focal symmetry is broken through a change in material properties. This change can be manifested as either a change in refractive index and/or a change in the nonlinear susceptibility of the specimen. Conversely, third harmonic production is inhibited in those cases when the axial focal symmetry is unbroken. This property can be effectively exploited to produce volumetric images of biological and non-biological specimens. The third harmonic is localized to interfaces within the specimen making it possible to produce optical sections in much the same manner as in confocal, and two-photon fluorescence microscopy. Unlike these techniques however, no exogenous fluorophore is necessary for labeling - the signal is produced by endogenous interfaces.