Modern space and astronomical telescopes come with increasingly demanding image performance requirements, and as a result apertures keep growing while surfaces depart further from a simple spherical shape. Meeting these requirements takes accurate and efficient metrology. For space telescopes, to maintain optical performance in orbit, in situ and real time wavefront measurement under the space environment is essential. This talk introduces optical testing techniques developed for space optics, starting with interferometry, still the most widely used method for high precision surface and wavefront measurement, then moving to deflectometry, which enables full aperture testing of large optical surfaces, and phase retrieval, which recovers wavefront information directly from focal plane images without additional metrology hardware. Each technique suits a different testing scenario, from ground based fabrication to on orbit performance monitoring, together illustrating the range of approaches needed to support the next generation of space telescopes.