We use third harmonic generation (THG) microscopy to image waveguides and single-shot structural modifications produced in bulk glass using femtosecond laser pulses. THG microscopy reveals the internal structure of waveguides written with a femtosecond laser oscillator, and gives a three-dimensional view of the complicated morphology of the structural changes produced with single, above-threshold femtosecond pulses. We find that THG microscopy is as sensitive to refractive index change as differential interference contrast microscopy, while also offering the three-dimensional sectioning capabilities of a nonlinear microscopy technique. It is now possible to micromachine three-dimensional optical devices and to image these structures in three dimensions, all with a single femtosecond laser oscillator.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.