The Advanced Mirror Technology Development (AMTD) project was a 6-year effort to mature technologies required to enable 4-m-or-larger monolithic or segmented ultraviolet/optical/infrared space-telescope primary-mirror assemblies for general astrophysics and exoplanet missions. AMTD used a science-driven systems-engineering approach. Starting with science requirements, engineering specifications were derived for the primary mirror aperture diameter, areal density, surface error, and stability. The most impactful specification may be 10 pm per 10-min wavefront stability. Advances were made to six key technologies: (1) fabricating large-aperture low-areal-density high-stiffness mirror substrates; (2) designing support systems; (3) correcting mid/high-spatial frequency figure error; (4) mitigating segment edge diffraction; (5) phasing segment-to-segment gaps; and (6) validating integrated models. AMTD successfully demonstrated a process to make substrates as large as 1.5 m and as thick as 40 cm by stacking multiple core elements and low-temperature fusing them together. To help predict on-orbit performance and assist in architecture trade studies, integrated models were created for two mirror assemblies (1.5-m Ultra-Low-Expansion (ULE^®) mirror fabricated by AMTD partner Harris Corp. and 1.2-m Zerodur^® mirror owned by Schott North American). X-ray-computed tomography was used to construct the 1.5-m ULE^® mirror’s “as-built” model. These models were validated by testing full- and subscale components in relevant thermovacuum environments.