Recent years have shown a dramatic growth of research activity in searching of materials that exhibit nonlinear optical (NLO) behaviours. Later investigations have shown that molecule-based organic materials and biomaterials possess advantages in NLO characteristics. This thesis is devoted to the theoretical and experimental study of the second-order nonlinear optical properties of native and modified bacteriorhodopsins. A hybrid quantum mechanical/molecular mechanical-configuration interaction (QM/MM-CI) method has been used to determine the nonlinear behaviour of modified BR. The calculated structures are the ground state and M1 state of wild type BR, several BR variants, the BR mutants W86F, W189F, W182F, W138C, Y185F, and Y83F as well as a photoreceptor NpSRII. The first hyperpolarizability of two available BR mutants W86F and D85C has been determined by means of the HRS technique. The experimentally determined first hyperpolarizability value for the mutant W86F is in good agreement with that obtained theoretically. Simultaneously with the time-resolved absorption spectroscopy measurements, the time-dependent HRS measurements were performed. Changes in the HRS signal during the photocycle were detected. The kinetics of time-resolved HRS and flash spectroscopy measurements of BR and the BR mutants T46C, R225C, R227C, D96C, and D85C were compared. The obtained kinetic constant are comparable with those determined by means of absorption spectroscopy. However, an unexpected decrease in the HRS intensity during the M to N transition was observed. The program "MultiFit" (written in the Delphi environment) as well as data acquisition software (written in the C environment) can be found online at: http://repositorium.uni-osnabrueck.de/bitstream/urn:nbn:de:gbv:700-2008020119/4/MultiFit.zip and: http://repositorium.uni-osnabrueck.de/bitstream/urn:nbn:de:gbv:700-2008020119/5/TRAS.zip