A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events. Examples of chain reactions include:

• The neutron-fission chain reaction: a neutron plus a fissionable atom causes a fission resulting in a larger number of neutrons than was consumed in the initial reaction. This reaction could continue if the number of neutron produced in a single reaction is capable of producing another fission. If not the reaction will stop. If the number of fissions produced is more than one then the reaction is said to be supercritical and the number of fissions would increase exponentially. This is the principle for an atomic bomb.
• For example in a chemical reaction every step of H₂ + Cl₂ chain reaction consumes one molecule of H₂ or Cl₂, one free radical H· or Cl· producing one HCl molecule and another free radical.
• Electron avalanche process: Collisions of free electrons in a strong electric field releasing "new" electrons to undergo the same process in successive cycles.
• A cascading failure, a failure in a system of interconnected parts, for example a power transmission grid, where the service provided depends on the operation of a preceding part, and the failure of a preceding part can trigger the failure of successive parts.
• Polymerase chain reaction, a technique used in molecular biology to amplify (make many copies of) a piece of DNA by in vitro enzymatic replication using a DNA polymerase.