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A comprehensive experimental and theoretical investigation of dispersion of the linear and nonlinear optical susceptibilities for amino acid L-alanine single crystals is reported. The state-of-the-art full potential linear augmented plane wave method, within a framework of the density functional theory was applied. The atomic positions from X-ray diffraction have been optimized so that the force on each atom is around 1 mRy au À1 . This relaxed geometry has been used for the theoretical calculations. The complex dielectric susceptibility dispersion, its zero-frequency limit and the birefringence of amino acid L-alanine single crystals were studied. The crystal exhibits a large uniaxial dielectric anisotropy resulting in a significant birefringence. The calculated birefringence at static limit is 0.072 and 0.074 at l ¼ 1064 nm (corresponding to 1.165 eV) in good agreement with the measured value (0.073). We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: c (2) 123 (u), c (2) 231 (u) and c (2) 312 (u). The calculated second order susceptibility tensor components |c (2) 123 (u)|, |c (2) 231 (u)|, and |c ( 2) 312 (u)| at l ¼ 1064 nm are compared with those obtained from our measurements performed using the 25 ps Nd:YAG pulsed laser at l ¼ 1064 nm. Our calculations are in reasonably good agreement with our experimental data. In addition we have calculated the microscopic second order hyperpolarizability, b 123 , vector component along the principal dipole moment directions for the dominant component c (2) 123 (u) and it is found to be 0.21 Â 10 À21 pm V À1 in the static limit and 0.27 Â 10 À21 pm V À1 at 1.165 eV (l ¼ 1064 nm) in comparison with our measured value (0.31 Â 10 À21 pm V À1 ) at l ¼ 1064 nm. Additional study of the second order susceptibilities versus the external laser treatment is performed.