CMOS (complementary Metal Oxide Semconductor)/SOI(sillcon on Insulator) devices are very promising for low-power and high-speed VLSI applications. Fundamental matters regarding SOI devices, such as a kind of SOI substrates, features of SOI CMOS structures, and two kinds of operation modes in SOI MOSFETs,i.e., fully-depleted and partially-depleted modes, are explamed, and the reason why CM0S/SOI is effective for low-power and high-speed LSIs is described. Floating body effects in SOI MOSFFTs and some proposed methods to prevent the effects are explained, and hot carrier reliability in ultra-thin SOI MOSFETs are also described. They are most important issue from the view point of device operation. Dynamic threshold-voltage MOSFFT, which is suitable for ultra-low voltage LSIs, is introduced . Finally,the effectiveness of SOI devices is veriied based on some expermental LSIs.