Optogenetics makes possible the control of neural activity with light. In this paper, I explore how the development of this experimental tool has brought about methodological and theoretical advances in the neurobiological study of memory. I begin with Semon’s (1921) distinction between the engram and the ecphory, explaining how these concepts present a methodological challenge to investigating memory. Optogenetics provides a way to intervene into the engram without the ecphory that, in turn, opens up new means for testing theories of memory error. I focus on a series of experiments where optogenetics is used to study false memory and forgetting. I conclude with discussion of the recent discovery of “silent engrams” (e.g., Roy, Muralidhar, Smith, & Tonegawa, 2017) using optogenetics and the way in which these results create further opportunities and challenges for engram theory.