  Many objects discovered by LIGO and Virgo are peculiar because they fall in a
mass range which in the past was considered unpopulated by compact object.
Given the significance of the astrophysical implications, it is important to
first understand how their masses are measured from gravitational-wave signals.
How accurate is the measurement? Are there elements missing in our current
model which may result in a bias? This chapter is dedicated to these questions.
In particular, we will highlight several astrophysical factors which are not
included in the standard model of GW sources but could result in a significant
bias in the estimation of the mass. These factors include strong gravitational
lensing, the relative motion of the source, a nearby massive object, and a
gaseous background.
