Achieving both high resolution and large collection area in the next generation of x-ray telescopes requires highly accurate shaping of thin mirrors, which is not achievable with current technology. Ion implantation offers a promising method of modifying the shape of mirrors by imparting internal stresses in a substrate, which are a function of the ion species and dose. This technique has the potential for highly deterministic substrate shape correction using a rapid, low cost process. Wafers of silicon and glass (D-263 and BK-7) have been implanted with Si+ ions at 150 keV, and the changes in shape have been measured using a Shack-Hartmann metrology system. We show that a uniform dose over the surface repeatably changes the spherical curvature of the substrates, and we show correction of spherical curvature in wafers. Modeling based on experiments with spherical curvature correction shows that ion implantation could be used to eliminate higher-order shape errors, such as astigmatism and coma, by using a spatially-varying implant dose. We will report on progress in modelling and experimental tests to eliminate higher-order shape errors. In addition, the results of experiments to determine the thermal and temporal stability of implanted substrates will be reported.