Strong field approximation (SFA) is a theoretical technique to describe electron motion in intense laser fields. It is becoming a standard framework to understand ultrafast atomic and molecular processes in intense laser fields where the field-driven motion of electron wavepackets plays crucial roles. The most striking feature of SFA is the capability to describe non-perturbative strong-field processes in an intuitive as well as a quantitative way. In this paper, I will review the framework of SFA that naturally results in the classical-quantum duality, and its applications to attosecond sciences, especially to high harmonic generation, two-color photoionization for attosecond measurement, and tomographic imaging of molecular orbitals.
