During the last 25 years (after the growth of the first pseudomorphic GeSi strained layers on Si by Erich Kasper in Germany) we have seen a steady accu- mulation of new materials and devices with enhanced performance made pos- sible by strain. 1989-1999 have been very good years for the strained-Iayer- devices. Several breakthroughs were made in the growth and doping technology of strained layers. New devices were fabricated as a results of these break- throughs. Before the advent of strain layer epitaxy short wavelength (violet to green) and mid-IR (2 to 5 f. Lm) regions of the spectrum were not accessi- ble to the photonic devices. Short wavelength Light Emitting Diodes (LEDs) and Laser Diodes (LDs) have now been developed using III-Nitride and II-VI strained layers. Auger recombination increases rapidly as the bandgap narrows and temperature increases. Therefore it was difficult to develop mid-IR (2 to 5 f. Lm range) lasers. The effect of strain in modifying the band-structure and suppressing the Auger recombination has been most spectacular. It is due to the strain mediated band-structure engineering that mid-IR lasers with good per- formance have been fabricated in several laboratories around the world. Many devices based on strained layers have reached the market place. This book de- scribes recent work on the growth, characterization and properties o(compound semiconductors strained layers and devices fabricated using them.