The idea that memory is stored in the brain as physical alterations goes back at least as far as Plato, but further conceptualization of this idea had to wait until the 20th century when two guiding theories were presented: Richard Semon’s “engram theory” developed in the early 20th century, and Donald Hebb’s “synaptic plasticity theory” in 1949. While a large number of studies have been conducted since, each supporting some aspect of these theories, they all fell short of providing integrative evidence for the existence of engram cells and circuits as defined by the theories. During the past few years, the advent of transgenics, optogenetics, and other technologies has allowed neuroscientists to identify memory engram cells. Furthermore, engram engineering technology is allowing neuroscientists to implant new memory in the mouse brain.