Strained‐layer Si1−xGex/Si heterostructures have created a great deal of interest due to the potential of integration with the conventional silicon very large scale integrated technology. With the current advances in silicon molecular beam epitaxy (Si‐MBE) and other low‐temperature epitaxial techniques, many Si1−xGex/Si heterojunction devices have been realized. For example, among those realized are avalanche photodiodes, modulation‐doped field‐effect transistors, heterojunction bipolar transistors, and more recently, resonant tunneling structures, hot‐carrier transistors, and quantum well metal‐oxide‐semiconductor field‐effect transistors. In this paper, several quantum size effects in strained Si1−xGexlayers and their potential in device applications will be reviewed. Among those to be discussed are resonant tunneling, miniband transport, and intersubband absorption in Si1−xGex/Si superlattice structures, optical properties of monolayer SimGensuperlattices, and observation of large Stark effect associated with interband transition between quantized states in Si1−xGex/Si quantum well structures.