  Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) has the largest
predicted cross-section of all low-energy neutrino couplings. However, as a
neutral-current interaction, the only experimental signature of CE$\nu$NS is a
low-energy nuclear recoil, which made its detection challenging. CE$\nu$NS
remained unobserved for over four decades. This thesis describes the experiment
that resulted in a CE$\nu$NS observation at a 6.7-sigma confidence level. A
low-background, 14.6-kg CsI[Na] scintillator was exposed to the neutrino
emissions from the Spallation Neutron Source at Oak Ridge National Laboratory.
Characteristic CE$\nu$NS signatures in energy and time, compatible with
predictions from the Standard Model, were observed in high signal-to-background
conditions. CE$\nu$NS provides new opportunities to study neutrino properties,
and enables the miniaturization of detectors.
