Macroporous silicon develops if silicon is anodically biased and electrochemically etched in hydrofluoric acid (HF). Especially for n-type silicon this is nowadays a well known process. Macroporous silicon with its high perfectness represents an ideal 2D photonic crystal exhibiting novel properties for the propagation of infrared light inside. On this page the pore formation will be considered in detail. In a second and third point the defect generation as well as modulations of the pore diameter are discussed.