The Goldstone theorem plays a crucial role in understanding spontaneous symmetry breaking in quantum field theory. Its implications are also found in condensed matter systems, such as superfluids and ferromagnets. The basic idea of the theorem is simple: spontaneous breaking of a continuous symmetry implies the existence of massless Goldstone bosons. In the article, after a brief review of definitions, a more precise formulation of the Goldstone theorem follows. Two proofs are presented, the first in the context of classical field theory and the second in quantum field theory. Finally, several examples of the application of the Goldstone theorem are provided, indicating the need for its generalizations.

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