  The discovery of cosmic acceleration has triggered a consistent body of
theoretical work aimed at modeling its phenomenology and understanding its
fundamental physical nature. In recent years, a powerful formalism that
accomplishes both these goals has been developed, the so-called effective field
theory of dark energy. It can capture the behavior of a wide class of modified
gravity theories and classify them according to the imprints they leave on the
smooth background expansion history of the Universe and on the evolution of
linear perturbations. The effective field theory of dark energy is based on a
Lagrangian description of cosmological perturbations which depends on a number
of functions of time, some of which are non-minimal couplings representing
genuine deviations from General Relativity. Such a formalism is thus
particularly convenient to fit and interpret the wealth of new data that will
be provided by future galaxy surveys. Despite its recent appearance, this
formalism has already allowed a systematic investigation of what lies beyond
the General Relativity landscape and provided a conspicuous amount of
theoretical predictions and observational results. In this review, we report on
these achievements.
