The development of efficient two-photon singlet oxygen sensitizers is addressed focusing on organic synthesis. Photophysical measurements were carried out on new lipophilic molecules, where two-photon absorption cross sections and singlet oxygen quantumyields were measured. Design principles for making efficient two-photon singlet oxygen sensitizers were then constructed from these results. Charge-transfer in the excited state of the prepared molecules was shown to play a pivotal role in the generationof singlet oxygen. This was established through studies of substituent effects on both the singlet oxygen yield and the two-photon absorption cross section, where it was revealed that a careful balancing of the amount of charge transfer present in theexcited state of the sensitizer is necessary to obtain both a high singlet oxygen quantum yield and a high two-photon cross section. An increasing amount of charge-transfer is beneficial for high two-photon absorption cross sections but iscounter-productive for singlet oxygen generation. The design principles obtained from the studies in lipophilic solvents were applied to synthesize water-soluble twophoton singlet oxygen sensitizers with the potential applicability for biological studies.Stability issues of these sensitizers were also addressed. Gas-phase measurements of the triplet quantum yield carried out on some of the sensitizers provided complementary information to the solution phase date and revealed that unsuccessful singletoxygen sensitizers had low triplet quantum yields. The synthesis of porphyrins with donor-acceptor architecture was investigated and it was shown that these biologically friendly molecules could not be brought to generate singlet oxygen in a two-photonirradiation scheme. Finally, the theoretical methods available for calculating two-photon absorption cross sections were applied to small molecules and provided insight on errors inherent in calculated cross sections.