Great progress was made in particle physics in the second half of the 20th Century, driven by the development of new types of accelerators and particle detectors. The combination of extensive experimental results and new theoretical insights led to the quark model, Quantum Chromodynamics, and Electroweak Unification. These form the basis of a remarkably successful theory of elementary particles, called the Standard Model. Although its predictions have been confirmed with good precision at present accelerator energies, this theory cannot be reliably extrapolated to the TeV energy region. Questions about spontaneous symmetry breaking and super symmetry have to be answered before there is a more complete and quantitative description of the physics at higher energies. Experimental programs are being prepared in the US and abroad to resolve these and other such issues.