We have measured the wavefront aberrations of fused silica and silicon microlenses using a Shack-Hartmann wavefront sensor system. The Shack-Hartmann sensor uses a combination of a microlens array and a CCD camera to measure wavefront local tilts with respect to a reference wavefront. Data reduction software then reconstructs the wavefront and expresses it in various forms such as Seidel or Zernike. We measured a series of our custom microlens arrays by placing a fiber source at a distance of one focal length behind the array to create a series of collimated beams from the individual lenslets. We then observed the quality of the collimated beams from single lenslets by using different aperture converters (for different sized lenslets) to expand the individual beams so that they filled a significant portion of the CCD area. For these microlens arrays, the P-V OPD was found to be less than λ/4 and the RMS wavefront error less than λ/20.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.