The possibilities and limitations of using two-photon excitation (TPE) in photodynamic therapy (PDT) have been studied. The study has been both theoretical and experimental, with special focus on the behaviour of two-photon excitation in turbid media. Parts of the two-photon absorption (TPA) theory, as well as that of PDT are given. In an experiment, the axial distribution of two-photon induced fluorescence in tissue phantom has been investigated. A considerable amount of publications in the area have also been reviewed. Various aspects of the topic is thoroughly discussed. The conclusions, based on the experiment as well as on the results of others, are that TPE PDT has an advantage of higher selectivity over single-photon excitation (SPE) photodynamic therapy, but that the possibilities to treat thick or deeply localised lesions are very limited for TPE PDT.

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