Since environmental pollution and energy shortage are becoming the big challenge for the new developing and developed world because of the industrial revolution, photocatalysis is an effective technology to solve the above problems. In this chapter, we briefly introduce the photocatalytic mechanism, the advantages of photocatalysis technology, and their applications. As the representative of photocatalyst, titanium dioxide is introduced in detail. The influence of different parameters (photocatalyst loading, pH, surface area and morphology, reaction temperatures, contaminant concentration, and calcination temperature of photocatalysts) on the degradation of pollutants is also introduced.