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��������������(��3��)(Ӣ��)����Ҫ��(n��i)�ݺ�(ji��n)�飺The last thirty years have seen an enormous advance in our understanding of the microscopic world. We now have a convincing picture of the fundamental struc-ture of observable matter in terms of certain point-like elementary particles. We also have a comprehensive theory describing the behaviour of and the forces between these elementary particles, which we believe provides a complete and correct description of nearly all non-gravitational physics.Matter, so it seems, consists of just two types of elementary particles: quarks and leptons. These are the fundamental building blocks of the material world, out of which we ourselves are made. The theory describ-ing the microscopic behaviour of these particles has,over the past decade or so, become known as the 'Standard Model', providing as it does an accurate account of the force of electromagnetism, the weak nuclear force ��responsible for radioactive decay��, and the strong nuclear force ��which holds atomic nuclei together��. The Standard Model has been remarkably successful; upuntil a year or two ago all experimental tests have verified the detailed predictions of the theory.The Standard Model is based on the principle of 'gauge symmetry', which asserts that the properties and interactions of elementary particles are governed by certain fundamental symmetries related to familiar conservation laws.