The black hole binaries detected so far by the LIGO-Virgo Collaboration have been “clean" sources of gravitational waves, since the binary dynamics were solely determined by gravity. Next generation gravitational-wave detectors, such as LISA, will in addition be sensitive to a number of “dirty” sources, affected by matter and the environment. In this talk, I will analyse in some detail two examples. In the first, I will consider black hole binaries located in active galactic nuclei (AGN binaries). In the LISA band, these binaries could be affected by a long list of environmental effects, from dynamical friction to accretion, from acceleration/Doppler to strong lensing. In the second example, I will consider compact bodies spiralling into the supermassive black hole at the centre of a galactic nucleus (an extreme-mass-ratio inspiral, or EMRI), and interacting with its accretion disk.