A new era of TiO_2 heterogeneous photocatalysis took its beginning in 1972 as Fujishima and Honda discovered water splitting on TiO_2 electrodes photocatalytically. This article describes TiO_2 crystal structures, photocatalytic principles, and classified photocatalytic examples. Some selected results obtained from our experiments are presented as well to stress the importance of surface chemistry in thoroughly understanding the complexity of TiO_2 photocatalytic reactions.