  Small fluctuations around homogeneous and isotropic expanding backgrounds are
the main object of study in cosmology. Their origin and evolution is sensitive
to the physical processes that happen during inflation and in the late
Universe. As such, they hold the key to answering many of the major open
questions in cosmology. Given a large separation of relevant scales in many
examples of interest, the most natural description of these fluctuations is
formulated in terms of effective field theories. This was the main avenue for
many of the important modern developments in theoretical cosmology, which
provided a unifying framework for a plethora of cosmological models and made a
clear connection between the fundamental cosmological parameters and
observables. In this review we summarize these results in the context of
effective field theories of inflation, large-scale structure, and dark energy.
