  The low-frequency radio sky is dominated by the diffuse synchrotron emission
of our Galaxy and extragalactic radio sources related to Active Galactic Nuclei
and star-forming galaxies. This foreground emission is much brighter than the
cosmological 21 cm emission from the Cosmic Dawn and Epoch of Reionization.
Studying the physical properties of the foregrounds is therefore of fundamental
importance for their mitigation in the cosmological 21 cm experiments. This
chapter gives a comprehensive overview of the foregrounds and our current
state-of-the-art knowledge about their mitigation.
