The following article is an edited transcript based on the David Turnbull Lecture given by Eugene E. Haller (University of California, Berkeley) at the 2005 Materials Research Society Fall Meeting in Boston on November 29, 2005. The David Turnbull Lectureship is awarded to recognize the career of a scientist who has made outstanding contributions to understanding materials phenomena and properties through research, writing, and lecturing, as exemplified by the life work of David Turnbull. Haller was named the 2005 David Turnbull Lecturer for his “pioneering achievements and leadership in establishing the field of isotopically engineered semiconductors; for outstanding contributions to materials growth, doping and diffusion; and for excellence in lecturing, writing, and fostering international collaborations.”The scientific interest, increased availability, and technological promise of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies of isotopically controlled semiconductor crystals. This article reviews results obtained with isotopically controlled semiconductor bulk and thin–film heterostructures. Isotopic composition affects several properties, such as phonon energies, band structure, and lattice constant, in subtle, yet—for their physical understanding–significant ways. Large isotope-related effects are observed for thermal conductivity in local vibrational modes of impurities and after neutron transmutation doping. Spectacularly sharp photo-luminescence lines have been observed in ultrapure, isotopically enriched silicon crystals. Isotope multilayer structures are especially well suited for studies of simultaneous self-and dopant-diffusion. The absence of any chemical, mechanical, or electrical driving forces makes possible the study of an ideal random-walk problem, in which moving atoms go in random directions at random intervals. Isotopically controlled semiconductors may find applications in quantum computing, nanoscience, and spintronics.