The Standard Solar Model (or SSM) predicts most aspects of our Sun's behaviour (the relationship between mass, temperature, luminousity, colour, magnetic field, etc.) to great accuracy, but has one major problem; the number of neutrinos we actually observe emenating from the Sun is approximately 1/3 that which we expect from the SSM. This is what is known as the Solar Neutrino Problem, or SNP.

The most widely accepted theory to explain this discrepancy is the existence of neutrino oscillations, by which electron neutrinos (the type emitted by the Sun, and which can be observed by most neutrino observatories) will turn into other types of neutrinos (which cannot be detected by conventional methods) as they travel. The Sudbury Neutrino Observatory is a project which hopes to prove or disprove this theory, utilizing its ability to detect neutrinos of mu and tau types.