  Since the appearance of General Relativity, its intrinsec general covariance
has been very often misinterpreted as implying that physically meaningful
quantitities (and conclusions extracted from the theory) have to be absolutely
independent on observers. This incorrect point of view is sometimes expressed
by discarding the very concept of observer in the structure and applications of
the theory. As we shall stress in this essay, through some examples, the
concept of observer is as essential to General Relativity as it is to any
physical theory.
