Abstract : Active Optics is a rising discipline which won renown on the largest telescopes in the world. The current and next instrumentation projects already use Active Optics at each level of their conception. Active mirrors are combining an excellent optical quality with a versatility and a flexibility what make them essential for the realisation of more and more performant telescopes and instruments, in order to reach the future scientific objectives. Three Active Optics techniques, based on stress polishing and in situ stressing of mirrors, are the core of this work. These techniques are developed in the frame of three major projects for the astronomical community : 1) the deformable secondary mirror for Vlt, 2) the exoplanet research instrument Sphere for Vlt and 3) the project of wide-eld multi-object spectrograph Eagle for the future European-Elt. Based on thin plates elasticity theory, analytical models allow defining loadcases and thickness distributions for the mirrors substrates to be deformed. Finite element analysis are a major tool to optimise and validate the proposed techniques. Several links are used and created between elasticity, aberration theory and spectral analysis, in order to evaluate the optical quality of the mirrors surfaces deformations. Three manufacturing process are developed from end to end in order to produce a large convex hyperbolic thin shell, toric mirrors of excellent optical quality and variable toric mirrors of high dynamic, the common factor being the excellent optical quality of the surface.