  In this article we review the current state of the field of solar neutrinos,
including flavour oscillations, non-standard effects, solar models, cross
section measurements, and the broad experimental program thus motivated and
enabled. We discuss the historical discoveries that contributed to current
knowledge, and define critical open questions to be addressed in the next
decade. We discuss the state of the art of standard solar models, including
uncertainties and problems related to the solar composition, and review
experimental and model solar neutrino fluxes, including future prospects. We
review the state of the art of the nuclear reaction data relevant for solar
fusion in the proton-proton chain and carbon-nitrogen-oxygen cycle. Finally, we
review the current and future experimental program that can address outstanding
questions in this field.
