Strong field approximation (SFA), or Keldysh-Faisal-Reiss theory is a physical model, started in 1964 by the Russian physicist Keldysh, is currently used to describe the behavior of atoms (and molecules) in intense laser fields.
SFA is the starting theory for discussing both high harmonic generation and attosecond pump-probe interaction with atoms.
The main assumption made in SFA is that the free-electron dynamics is dominated by the laser field, while the Coulomb potential is regarded as a negligible perturbation.
This fact re-shapes equation  into:
where,  is the Volkov Hamiltonian, here expressed for simplicity in the velocity gauge, with ,  , the electromagnetic vector potential.
At this point, to keep the discussion at its basic level, lets consider an atom with a single energy level , ionization energy  and populated by a single electron (single active electron approximation).
We can consider the initial time of the wave function dynamics as , and we can assume that initially the electron is in the atomic ground state .
So that,
: and 
Moreover, we can regard the continuum states as plane-wave functions state, .
This is a rather simplified assumption, a more reasonable choice would have been to use as continuum state the exact atom scattering states.
The time evolution of simple plane-wave states with the Volkov Hamiltonian is given by:
here for consistency with Eq.  the evolution has already been properly converted into the length gauge.
As a consequence, the final momentum distribution of a single electron in a single-level atom, with ionization potential , is expressed as:
where,
is the dipole expectation value (or transition dipole moment), and
is the semiclassical action.
The result of Eq.  is the basic tool to understand phenomena like:
* The high harmonic generation process, which is typically the result of strong field interaction of noble gases with an intense low-frequency pulse,
* Attosecond pump-probe experiments with simple atoms. 
* The debate on tunneling time.