  Nonreciprocal components, such as isolators and circulators, are crucial
components for photonic systems. In this article we review theoretical and
experimental progress towards developing nonreciprocal photonic devices based
on dynamic modulation. In particular, we focus on approaches that operate at
optical wavelengths and device architectures that have the potential for
chip-scale integration. We first discuss the requirements for constructing an
isolator or circulator using dynamic modulation. We review a number of
different isolator and circulator architectures, including waveguide and
resonant devices, and describe their underlying operating principles. We then
compare these device architectures from a system-level performance perspective,
considering how their figures of merit, such as footprint, bandwidth,
isolation, and insertion loss scale with respect to device parameters.
