A simple model for above threshold ionization is presented in this work. It is based on modeling the pulsed laser beam involved in ionization as a cylinder consisting of elementary cells whose volumes are very small compared to the pulse volume. A cell is occupied by a number of photons determined probabilistically and found to follow the Poisson distribution. During ionization, resulting from the application of the laser field, a typical atom finds itself in one of the cells and one of its electrons absorbs the photon energy. The electron will be detached from its atom and will exit the interaction volume (the laser beam) if the energy contained in the particular cell is high enough. The model predicts all possible energies of the electrons exiting the beam and produces energy spectra associated with these electrons. While most of the other available models make only qualitative comparisons, we are able to make both qualitative and quantitative comparisons with experimental data. / Master of Science