Porous silicon samples were grown under various anodization conditions on {100} oriented n-silicon substrates. Scanning and transmission electron microscope observations have shown that the microstructure of porous silicon primarily falls into three categories, which can be classified as single layer, double layer, and large macropore types. These three types differ not only in their microstructural details, but also in luminescence properties. In all three types of samples, the inner surface of pores created by anodization is coated with a low density amorphous material, which contained silicon and oxygen. In addition, all the pore surfaces were roughened by anodization, leaving nanoscale crystalline silicon asperities.