Several decades of a rich and diverse program of experimental neutrino measurements have provided an increasingly clearer picture of the elusive neutrino sector, and uncovered physics not predicted by the Standard Model (SM), such as the existence of nonzero neutrino masses implied by the surprising discovery of neutrino flavor mixing. This foundational discovery represented a welcome resolution to decades-long experimental anomalies associated with solar and atmospheric neutrino measurements.

Alongside this foundational discovery, experimental neutrino anomalies have been observed that still remain unresolved, and have served as primary drivers in the development of a vibrant short-baseline neutrino program, and in the launch of a multitude of complementary probes within a large variety of other experiments. Two of these anomalies arise from the apparent oscillatory appearance of electron (anti)neutrinos in relatively pure muon-(anti)neutrino beams originating from charged-pion decay-at-rest, specifically the LSND Anomaly, and from charged-pion decay-in-flight, the MiniBooNE Low-Energy Excess. Two other anomalies are associated with an overall normalization discrepancy of electron (anti)neutrinos expected both from conventional fission reactors, the Reactor Neutrino Anomaly, and in the radioactive decay of Gallium-71, the Gallium Anomaly. In these two latter cases, no oscillatory signature is observed, but the overall normalization deficit can be ascribed to rapid oscillations that are averaged out and appear as an overall deficit.

Presently under construction in Lund, Sweden, the European Spallation Source (ESS) will be the world's brightest neutron source. As such, it has the potential for a particle physics program with a unique reach and which is complementary to that available at other facilities. This paper describes proposed particle physics activities for the ESS. These encompass the exploitation of both the neutrons and neutrinos produced at the ESS for high precision (sensitivity) measurements (searches).

ESSnuSB is a design study for an experiment to measure the CP violation in the leptonic sector at the second neutrino oscillation maximum using a neutrino beam driven by the uniquely powerful ESS linear accelerator. The reduced impact of systematic errors on sensitivity at the second maximum allows for a very precise measurement of the CP violating parameter. This review describes the fundamental advantages of measurement at the second maximum, the necessary upgrades to the ESS linac in order to produce a neutrino beam, the near and far detector complexes, and the expected physics reach of the proposed ESSnuSB experiment, concluding with the near future developments aimed at the project realization.