This module describes at an introductory and non mathematical level our current knowledge of the smallest known particles and the fundamental forces that act between them. The properties of the basic constituents of matter, namely quarks, leptons and bosons are described together with the experimental techniques that have been used to produce our current understanding. The fundamental forces are discussed in terms of the exchange model of forces. The properties of these forces are investigated, the bosonic carriers of each force are identified and the ideas behind the successful unification of the electromagnetic and weak forces are discussed. The connection between conservation laws and symmetry are illustrated and Feynman diagrams are introduced to represent particle interactions and decays. Some major experiments are investigated in detail and used as case studies to illustrate more general concepts of particle detection and identification. These experiments include those that provided evidence for quarks, the discovery of the W and Z bosons, precise measurements of the Z boson and underground experiments which investigate properties of the various types of neutrino.