The study of semiconductor heterostructures started more than forty years ago. In the 1980s this area of research moved to the forefront of semiconduc tor physics, largely due to progress in growth technologies which are now capable of producing ultrathin layers (up to a few monolayers) of different semiconductor materials. The availability of structures with nearly ideal, well-controlled properties has made semiconductor heterostructures a test ing ground for solid-state physics. These structures have had a profound impact on basic research in semiconductor physics by opening new possibil...