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Reference systems and gravity are topics relevant not only to Geodesy, but also to several other fields of research. The Theory of General Relativity is based on a geometric view of gravity, and therefore coordinates and gravity become inseparable. As the precision of the geodetic measurements with satellite techniques increases, the classical measurement modeling progressively gives way to a treatment in which concepts of Relativity take on an ever greater importance. In this paper, the differences between the classical and relativistic concepts of space-time in the presence of gravity, and the corresponding different role of the coordinates are first analyzed. Then, we move on to the application plan, facing the three major - for the moment - examples of the presence of General Relativity in the representation of geodetic measurements: secular and periodic modification of the time delivered by on-board atomic clocks in GNSS satellites; deviation of the trajectory of light rays in a gravitational field, and corresponding delay in arrival time; and finally dragging of the inertial systems due to the rotation of the central body (Lense Thirring effect).