Bell's theorem represents a significant advance in understanding the conceptual foundations of quantum mechanics. It shows that all local theories of natural phenomena formulated within the framework of realism can be tested using a single experimental arrangement. These theories must make predictions that significantly differ from those of quantum mechanics. Experimental results have refuted the predictions of these theories and favored those of quantum mechanics. This has philosophical implications: either one must abandon the realistic philosophy of most working scientists, or dramatically revise our concept of space-time. The theorem was derived by John F. Clauser and Abner Shimony, building on earlier work by John Bell. The proof involves showing that deterministic local hidden-variables theories cannot reproduce all the statistical predictions of quantum mechanics. Clauser and Horne later provided an experimental test of this theorem, which has been performed in various experiments involving photons and particles. These experiments have generally agreed with quantum mechanics, supporting the thesis that realism or locality must be abandoned. The historical background includes Einstein, Podolsky, and Rosen's (EPR) argument for the incompleteness of quantum mechanics, which was later critiqued by Bohr. Bell's theorem was derived from EPR's argument, showing that local realistic theories are incompatible with quantum mechanics. The theorem has inspired various experiments, most of which have yielded results consistent with quantum mechanics but inconsistent with local realistic theories. The review covers the historical context, the EPR argument, Bell's theorem, and its experimental tests. It discusses the requirements for a general experimental test, the cascade-photon experiments, and related experiments involving positronium annihilation and proton-proton scattering. The evaluation of experimental results and future prospects are also discussed.Bell's theorem represents a significant advance in understanding the conceptual foundations of quantum mechanics. It shows that all local theories of natural phenomena formulated within the framework of realism can be tested using a single experimental arrangement. These theories must make predictions that significantly differ from those of quantum mechanics. Experimental results have refuted the predictions of these theories and favored those of quantum mechanics. This has philosophical implications: either one must abandon the realistic philosophy of most working scientists, or dramatically revise our concept of space-time. The theorem was derived by John F. Clauser and Abner Shimony, building on earlier work by John Bell. The proof involves showing that deterministic local hidden-variables theories cannot reproduce all the statistical predictions of quantum mechanics. Clauser and Horne later provided an experimental test of this theorem, which has been performed in various experiments involving photons and particles. These experiments have generally agreed with quantum mechanics, supporting the thesis that realism or locality must be abandoned. The historical background includes Einstein, Podolsky, and Rosen's (EPR) argument for the incompleteness of quantum mechanics, which was later critiqued by Bohr. Bell's theorem was derived from EPR's argument, showing that local realistic theories are incompatible with quantum mechanics. The theorem has inspired various experiments, most of which have yielded results consistent with quantum mechanics but inconsistent with local realistic theories. The review covers the historical context, the EPR argument, Bell's theorem, and its experimental tests. It discusses the requirements for a general experimental test, the cascade-photon experiments, and related experiments involving positronium annihilation and proton-proton scattering. The evaluation of experimental results and future prospects are also discussed.