  Light can be used as a probe to explore the structure of space-time: this is
usual in astrophysical and cosmological tests, however it has been recently
suggested that this can be done also in terrestrial laboratories. Namely, the
GINGER project aims at measuring post-Newtonian effects, such as the
gravito-magnetic ones, in an Earth based laboratory, by means of a ring lasers
array. Here, we first review the theoretical foundations of the Sagnac Effect,
on which ring lasers are based, and then we study the Sagnac Effect in a
terrestrial laboratory, emphasizing the origin of the gravitational
contributions that GINGER aims at measuring. Moreover, we show that accurate
measurements allow to set constraints on theories of gravity different from
General Relativity. Eventually, we describe the experimental setup of GINGER.
