First-principles studies on TiO₂/H₂O interfaces, representative photocatalytic systems, are reviewed. First I show our recent first-principles molecular dynamics study on anatase TiO₂/H₂O interfaces, with emphasis on the difference of calculation techniques and results from those in the previous studies. Then, arguments on water adsorption mode (molecular or dissociative etc.) on TiO₂ rutile (110) surface are discussed. Finally, with our recent results, I introduce the difference of surface nanostructure stability between vacuum and aqueous conditions, which is relevant to the difference between dry and wet processes.