Nuclear power stations are the major source of human generated neutrinos. An average plant may generate over 50,000 neutrinos per second. Particle accelerators are another source.

Neutrinos are produced as a result of natural background radiation.

Atmospheric neutrinos result from the interaction of cosmic rays with atoms within the Earth's atmosphere, creating showers of particles including neutrinos.

Solar neutrinos originate from the nuclear fusion powering the Sun and other stars.

Neutrinos are an important product of supernovas. Most of the energy produced in supernovas is radiated away in the form of an immense burst of neutrinos, which are produced when protons and electrons in the core combine to form neutrons. The first experimental evidence of this phenomenon came in the year 1987, when neutrinos coming from the supernova 1987a were detected. In such events, the densities at the core becomes so high (10¹⁴ g/cm³) that interaction between the produced neutrinos and surrounding stellar matter becomes significant. It is thought that neutrinos would also be produced from other events such as the collision of neutron stars.

Because neutrinos interact so little with matter, it is thought that a supernova's neutrino emissions carry information about the innermost regions of the explosion. Much of the visible light comes from the decay of radioactive elements produced by the supernova shock wave, and even light from the explosion itself is scattered by dense and turbulent gases. Neutrinos, on the other hand, pass through these gases, providing information about the supernova core (where the densities were large enough to influence the neutrino signal). Furthermore, the neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays or radio waves. The exact time delay is unknown, but for a Type II supernova, astronomers expect the neutrino flood to be released seconds after the stellar core collapse, while the first electromagnetic signal may be hours or days later. The SNEWS (http://snews.bnl.gov) project uses a network of neutrino detectors to monitor the sky for candidate supernova events; it is hoped that the neutrino signal will provide a useful advance warning of an exploding star.

It is thought that the cosmic background radiation left over from the Big Bang includes a background of low energy neutrinos. In the 1980s it was proposed that these may be the explanation for the dark matter thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: we know they exist. However, they also have serious problems. From particle experiments, it is known that neutrinos tend to be hot, i.e. move at speeds close to the speed of light—hence this scenario was also known as hot dark matter. The problem is that being hot and fast moving, the neutrinos would tend to spread out evenly in the universe. This would tend to cause matter to be smeared out and prevent the large galactic structures that we see.