The current status of our knowledge of the structure of matter at the fundamental level and of the laws governing the microworld, encompassed in the Standard Model, is reviewed. The basic features of electromagnetic, weak and strong forces acting among the fundamental building blocks of matter, quarks and leptons, are all shown to result from a single guiding principle: the requirement for local gauge invariance. This principle is also behind the attempts at unifying these forces within the framework of the Grand Unified Theories. These theories, where quarks and leptons are merely different states of the same fundamental fermion, imply instability of protons and neutrons, the feature that may have played a crucial role at the early stage of the Big Bang. For about 30 years, much attention has also been devoted to theoretical development and experimental testing of the idea of supersymmetry. This idea, which relates fermions to bosons, assumes that all particles of the Standard Model have partners which differ in spin by 1/2. So far no supersymmetric partners of quarks, leptons or gauge bosons have been found, but experiments at CERN should answer in the near future the question whether nature is, indeed, supersymmetric. Supersymmetry is also an integral part of the string theories, which go even further beyond the Standard Model and assume that the basic structures of matter are not one-dimensional particles, but two-dimensional strings, moving, moreover, in space with more than three dimensions. The idea that the laws of nature may be simple when formulated in space with extra dimensions is the most fascinating consequence of the present particle theory since it provides the basic framework for unification of all forces, including gravity.