Urocanic acid (UCA):  is an epidermal chromophore, a metabolic deamination product of histidine. It accumulates in excess on the upper layer of our skin in its trans-form. However, it isomerizes to cis-form upon exposure of UV radiation. This in turn may lead to an immunosuppressant reaction, linked to UV-induced skin cancer. Very little is known about its complex photophysics. This is PhD project of Jonas Danielsson. We first carried out MD simulations [Danielsson, Söderhäll & Laaksonen, Mol. Phys., 100, 1873 (2002)] and showed that the carboxylic group of the zwitterionic form could rotate freely in water solution. In general we found that explicit solvent effects and correct protonation form are crucial for description of the dynamics and relative energies of conformers, qualitatively changing the results from previous ab initio calculations corresponding to vacuum conditions. For the cis isomer, the intra-molecular hydrogen bond was frequently broken due to water molecules making it more floppy than the trans form. As a second step we have studied systematically its photochemistry in all biologically relevant ionic, rotameric and protomeric forms [Danielsson, Ulicny & Laaksonen, J. Am. Chem. Soc. 123, 9817 (2001)]. Again, inclusion of solvation effects was crucial. We also observed that the protonated trans form, found on the epidermis, gives rise to a transition, which coincides with the singlet oxygen production action spectrum maximum and thereby provides a possible link to photo aging. We carried out a separate study of gas-phase isomerization of UA where we explained the origin of the experimental supersonic jet spectroscopic results coming from two different isomers coexisting in the gas phase [Danielsson & Laaksonen, Chem. Phys. Lett., 370 625 (2003]. In subsequent paper we suggest a new model elegantly explaining the photochemistry of UCA over the entire UV region, including the trans-cis isomerization of the anionic form of UCA (found in the biological tissues of skin) in aqueous solution. The isomerization is found to be different from the classical cases of stilbene and retinal starting with an excitation to the lowest nπ* state to reach the intersection with ππ* state, leading finally to the photo-isomerization [Danielsson PhD thesis].