As alternative route to fabricate silicon nanostructures, wet etching processes such as electrochemical and electroless etching of silicon have attracted intensive interests recently, due to their simple cost-effective solution based process.    Electrochemical etching of silicon in hydrofluoric acid solutions generally results in either porous silicon formation or electro-polishing at low or at high current densities, respectively. In an intermediate regime, pores merged resulting in free standing silicon wires. Regular pore and wire array could be obtained if etching is preceded by a lithographic step where etch pits, initiating the pore growth, are formed at regular intervals. By using some tricks in pattern and applied current, silicon pyramid and sharp tip array were also obtained in our lab.   Different from electrochemical etching, electroless etching of silicon is a purely chemical etching process without using electrodes. It can produce aligned single-crystal Si nanowires by scratching the Si surface with Ag particles in oxidizing aqueous HF solution, and creation of nanometer scale pores in Si coated with metal particles.