  Modified gravitational wave propagation is a smoking gun of modifications of
gravity at cosmological scales, and can be the most promising observable for
testing such theories. The observation of gravitational waves (GW) in recent
years has allowed us to start probing this effect, and here we briefly review
two promising ways of testing it. We will show that, already with the current
network of detectors, it is possible to reach an interesting accuracy in the
estimation of the $\Xi_0$ parameter (that characterizes modified gravitational
wave propagation, with $\Xi_{0, {\rm GR}} = 1$) and with next generation
facilities, such as the Einstein Telescope, we can get a sub-percent
measurement.
