A neutron monitor is a ground-based detector designed to measure the number of high-energy charged particles striking the Earth's atmosphere from outer space. For historical reasons the incoming particles are called "cosmic rays", but in fact they are particles, predominantly protons and Helium nuclei. Most of the time, a neutron monitor records galactic cosmic rays and their variation with the 11-year sunspot cycle and 22-year magnetic cycle. Occasionally the Sun emits cosmic rays of sufficient energy and intensity to raise radiation levels on Earth's surface to the degree that they are readily detected by neutron monitors. They are termed "ground level enhancements" (GLE).

The neutron monitor was invented by University of Chicago Professor John A. Simpson in 1948. The "18-tube" NM64 monitor, which today is the international standard, is a large instrument weighing about 36 tons.

When a high-energy particle from outer space ("primary" cosmic ray) encounters Earth, its first interaction is usually with an air molecule at an altitude of 30 km or so. This encounter causes the air molecule to split into smaller pieces, each having high energy. The smaller pieces are called "secondary" cosmic rays, and they in turn hit other air molecules resulting in more secondary cosmic rays. The process continues and is termed an "atmospheric cascade". If the primary cosmic ray that started the cascade has energy over 500 MeV, some of its secondary byproducts (including neutrons) will reach ground level where they can be detected by neutron monitors.

Since they were invented by Prof. Simpson in 1948 there have been various types of neutron monitors. Notable are the "IGY-type" monitors deployed around the world during the 1957 International Geophysical Year (IGY) and the much larger "NM64" monitors (also known as "supermonitors"). All neutron monitors however employ the same measurement strategy that exploits the dramatic difference in the way high and low energy neutrons interact with different nuclei. (There is almost no interaction between neutrons and electrons.) High energy neutrons interact rarely but when they do they are able to disrupt nuclei, particularly heavy nuclei, producing many low energy neutrons in the process. Low energy neutrons have a much higher probability of interacting with nuclei, but these interactions are typically elastic (like billiard ball collisions) that transfer energy but do not change the structure of the nucleus. The exceptions to this are a few specific nuclei (most notably ¹⁰B and ³He) that quickly absorb extremely low energy neutrons, then disintegrate releasing very energetic charged particles. With this behavior of neutron interactions in mind, Professor Simpson ingeniously selected the four main components of a neutron monitor:

Neutron monitors measure by proxy the intensity of cosmic rays striking the Earth, and its variation with time. These variations occur on many different time scales (and are still a subject of research). The three listed below are examples:

In a process termed “solar modulation” the Sun and solar wind alter the intensity and energy spectrum of Galactic cosmic rays that enter the Solar System. When the Sun is active, fewer Galactic cosmic rays reach Earth than during times when the Sun is quiet. For this reason, Galactic cosmic rays follow an 11-year cycle like the Sun, but in the opposite direction: High solar activity corresponds to low cosmic rays, and vice versa.

The main advantage of the neutron monitor is its long-term stability making them suitable for studied of cosmic-ray variability through decades.

The most stable long-running neutron monitors are: Oulu, Inuvik, Moscow, Kerguelen, Apatity and Newark neutron monitors.