Multistep photoionization process is discussed concerning the Atomic Vapor Laser Isotope Separation applied to isotopes of heavy elements which has been carried out at Japan Atomic Energy Research Institute these 6 years. We present the studies related to the collection of basic spectroscopic data, selection of the most effective stepwise ionization, photodynamical characterization, excitation of hyperfine structure and laser beam propagation in absorbing medium. Techniques to determine the optical properties using stepwise photoionization, optimization techniques to find the most effective combination of laser wavelengths are also presented.