The evolution of semiconductor optical sources and photodetectors for optical communication systems is described. From rudimentary GaAsp–njunction emitters and silicon photodiodes in the mid-1960s, the paper charts progress towards the much more sophisticated multilayer laser structures and III–V avalanche photodetectors being made in the mid-1980s. Technological breakthroughs are identified, as well as some of the major problems that have had to be solved, such as well controlled epitaxial growth and device reliability.