Recent developments in strained layer epitaxial systems are reviewed. Their interest stems primarily from the additional degree of freedom that strained layers provide in the design of heterostructures and devices, which has led to device structures that can be tailored to a particular application with, in many cases, performances that are and out of reach with lattice-matched systems alone. With the advent of SiGe alloys, the concept of strained layers was extended to include the elemental semiconductors as well. Si/SiGe heterojunctions have provided a way to study quantum phenomena and explore heterojunction bipolar transistors (HBTs) and field-effect transistors (FETs). In optoelectronics, quantum well lasers with strained InGaAs active layers provide considerable reduced threshold current density. With coherently strained active layers based on GaAs, lasers with longevities superior to those with lattice-matched channels have been obtained.>