  This is an expository article about power operations and their connection
with the study of highly structured ring spectra. In particular, we discuss
Dyer-Lashof operations and their evolving role in the study of iterated loop
spaces, $E_n$-algebras, and $E_n$-ring spectra. We will make use of these
operations to show that structured ring spectra are heavily constrained. We
also discuss some ongoing directions for study.
  This is a preliminary version of a chapter written for the Handbook of
Homotopy Theory.
