Solar energy utilization through semiconductor photocatalysis has been intensitively investigated for both environmental and energy applications. Heterogeneous photocatalytic reactions are mostly surface chemical reactions and the surface properties of the semiconductor play a critical role in determining the reaction efficiencies and mechanisms. The surface properties of TiO₂, the most popular semiconductor photocatalyst, are related with various parameters that include pH, surface charge, surface hydroxyl group density, particle size, crystalline phase, surface defects, surface metal (e.g., Pt, Au) deposits, and adsorbates or surface complexes. TiO₂ surface can be actively modified by manipulating the above parameters in order to optimize or control the photocatalytic reactions. This talk will introduce and discuss various examples that show how surface modification influences the photocatalytic activities in relation with pollutant degradation and hydrogen production. The methods of surface modification of TiO₂ include dye sensitization, surface platinization, surface fluorination, surfactant adsorption, and nafion coating. The effects of surface modification are widely varying depending on the method and the kind of substrates. In particular, the simultaneously platinized and fluorinated TiO₂ exhibited unique behaviors in anoxic degardation of organic substrates and the simultaneous degradation of organic substrates and production of hydrogen. Understanding the relation between the surface property and the photocatalytic activity is essentially required to control the activity but not much is understood awaiting more elaborate studies at the molecular level.