  In this chapter, I describe bulk viscosity as a general concept, and then
focus on bulk viscosity in the dense matter present in compact objects. While
this review is focused on bulk viscosity in the conditions present in neutron
star mergers, I present a history of bulk viscosity research in dense matter,
from its role in damping radial oscillations in neutron stars through its
current applications in neutron star mergers. The majority of the chapter
consists of calculations of the bulk viscosity from Urca processes in generic
neutron-proton-electron ($npe$) matter, and then in dense matter containing
muons ($npe\mu$ matter) as well. I make several approximations in these
calculations to keep the focus on the concepts. More precise calculations exist
in the literature, to which I refer the reader. One concept I attempt to
elucidate is the thermodynamic behavior of a fluid element throughout an
oscillation and how that leads to bulk-viscous dissipation. I conclude with a
discussion of the recent research into the role of weak interactions and bulk
viscosity in neutron star mergers.
