A (SHORT) INTRODUCTION TO BLACK HOLES The general theory of relativity, proposed by A. Einstein in 1915 and described by the Einstein field equations, attributes to mass the capability to modify the surrounding space-time. The larger the mass, the stronger the curvature that a body causes on the space-time ___ an effect which consequently is more likely to be detected when observing very massive objects. This explains why astrophysics played a keyrole in testing the general theory of relativity. The curvature of space-time caused by the Sun (M=M☉ ~ 1030 kg) clearly manifests itself, e.g., in the anomalous rate of precession of the perihelion of Mercury's orbit. On a far higher mass scale, the space-time warp induced by clusters of galaxies (M ~ 1013 M☉) is revealed by the detection and the amplification of the light from background sources (gravitational lensing), which allows us to observe objects which otherwise would remain undetected.    The mass, however, is not the only important parameter in the study of the relativistic effects which a body produces on the surrounding environment. A spherical object having the mass and the radius of the Sun can deflect the light by a measurable, yet weak quantity. What would it happen if the same mass was instead concentrated within a sphere of a few kilometers radius? The gravitational force exerted on the light crossing the sphere would be much more intense. This simple argument can be expanded by considering the first exact solution to Einstein field equations, proposed by K. Schwarzschild in 1916, for the elementary case of a spherical, non-rotating and non-charged mass. Looking at this solution, one can deduce that, if a mass M is concentrated in a spherical volume with a radius smaller than rs = 2GM/c2 (where G is the universal gravitational constant and c the speed of light in vacuum; rs is referred to as the Schwarzschild radius), the exerted gravitational attraction is such that any body crossing the sphere would be inexorably sucked in. The basic idea behind the concept of black hole is the following ___ an object which is capable of trapping anything that passes close enough to it, even light. The surface which marks the point of no return is called an event horizon. In the case discussed by Schwarzschild, the event horizon is the surface of the sphere of radius rs, while in more complicated cases its geometry can be quite different. Note that the fundamental parameter characterizing a black hole is not an extreme mass, but rather an extraordinary density.    For several years, the common reaction to the concept of black holes was skepticism ___ they were generally considered rather a mathematical than a physical issue. Gradually, both the development of the theoretical background and the evidence provided by astronomical observations contributed to support black holes as a physical reality. Nowadays, the theory of black holes is integrated in both particle physics and astrophysics. The existence of stellar (M ~ 101 M☉) and supermassive black holes (M ~ 106 - 109 M☉) is supported by very convincing evidence. It is likely that black holes exist on intermediate (M ~ 103 M☉) and on much smaller mass scales (micro black holes, M ~ 10-7 M☉). Modern theories aiming at unifying gravity with the quantum theory even postulate the existence of quantum black holes (M ~ 10-19 M☉). Using the Schwarzschild radius, from a mass scale we can deduce a spatial scale. It results that the spatial range comprised by the physics of black holes is extremely wide - looking at the numbers above, something like 28 orders of magnitude! Apparently, the study of black holes has implications which are not con- ned to the microscopic world, nor to the sidereal distances typical in astrophysics. It embraces a variety of phenomena which very few other research elds can equal.