  In the present paper we study the Geodesic Deviation Equation (GDE) in the
modified $f(Q)$-gravity theories. The formulation of GDE in General Relativity
in the case of the homogeneous and isotropic
Friedman-Lema\^{i}tre-Robertson-Walker (FLRW) spacetime is briefly discussed
and then extended in modified $f(Q)$-gravity using its covariant counterpart.
The generalised Mattig relation is obtained. Finally, an equivalent expression
to the Dyer-Roeder equation in General Relativity in the case of $f(Q)$-gravity
is presented.

General Relativity and Quantum Cosmology