We have developed the adiabatic theory of ionization of an atom/molecule by an intense low-frequency laser pulse. The asymptotics of the solution to the time dependent Schrodinger equation and the photoionization momentum spectra are obtained from the Siegert states which are solutions to the time independent equation of the  target under a static field.We will present some illustrative calculations  which confirm that the adiabatic results  converge to the exact ones as the adiabatic parameter tends to zero.