Photoelectrochemical (PEC) etching is a simple and inexpensive wet etching approach that is widely used to fabricate porous gallium nitride (GaN) thin films. However, many fundamental issues on the etching mechanism and the optical response of the fabricated porous structure still remain unclear. In this book, PEC etched porous GaN thin films with a variety of morphologies such as circular, hexagonal, leaf like-, and honeycomb-like patterns were described in detail. The effects of semiconductor types, etching voltage and etching duration on the surface morphology and the optical response of the fabricated porous structure were discussed. Attenuated total reflection method which is very sensitive to the surface layer of the porous GaN was applied to extract the carrier concentration, porosity, and layer thicknesses or the porous layer. Through this book, a better understanding of the PEC etching of the porous GaN can be obtained.