We have developed several calculation methods for the ionization of atoms and molecules by strong and ultrashort laser pulses, based on the numerical solution of the time dependent Schrodinger equation (TDSE) in the momentum space. We have performed calculations within the strong field approximation (Volkov) and using iterative and direct methods for solving the TDSE. The investigated molecules are H-2(+) and H2O. In case of the ionization of diatomic molecules the interference effects in the ejected electron spectra due to the coherent addition of the waves associated to the electrons ejected from the vicinity of different nuclei were also analysed.