Abstract: The latest development of research on femtosecond nonlinear optics is introduced in this article. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high field physics. Implications include the generation of coherent harmonic radiation up to keV photon energies.