The III-V nitrides AlGaN has received a lot of attention recently as one of the important wide band gap semiconductor materials for fabricating optoelectronic components such as ultraviolet photodetectors, heterostructure field effect transistors, and high electron mobility transistors. But difficulties in realizing low-resistance ohmic contacts between metals and AlGaN have blocked their developments. By summering and analyzing the related papers published in recent years, the developments of ohmic contact formation, metallization scheme, alloying process and surface treatments for various AlGaN devices are presented.