Donald Olding Hebb (1904–1985) was a Canadian psychologist whose work has had a profound impact on cognitive neuroscience. He studied how brain cells (neurons) contribute to psychological processes. Hebb suggested, in a nutshell, that “neurons that fire together, wire together.” This famous rule is now widely considered one of the most important cellular mechanisms of learning and memory. Although Hebb made the suggestion in 1949, the biological techniques required to observe it were not available until the late 1960s.