  Progress in electron-beam spectroscopies has recently enabled the study of
optical excitations with combined space, energy and time resolution in the
nanometer, millielectronvolt and femtosecond domain, thus providing unique
access into nanophotonic structures and their detailed optical responses. These
techniques rely on $\approx$ 1-300 keV electron beams focused at the sample
down to sub-nanometer spots, temporally compressed in wavepackets a few
femtoseconds long, and in some cases controlled by ultrafast light pulses. The
electrons undergo energy losses and gains, also giving rise to
cathodoluminescence light emission, which are recorded to reveal the optical
landscape along the beam path. This review portraits these advances, with a
focus on coherent excitations, emphasizing the increasing level of control over
the electron wave functions and ensuing applications in the study and
technological use of optically resonant modes and polaritons in nanoparticles,
2D materials and engineered nanostructures.

Optics;Materials Science;Instrumentation and Detectors