  The large scale structures of the universe will likely be the next leading
source of cosmological information. It is therefore crucial to understand their
behavior. The Effective Field Theory of Large Scale Structures provides a
consistent way to perturbatively predict the clustering of dark matter at large
distances. The fact that baryons move distances comparable to dark matter
allows us to infer that baryons at large distances can be described in a
similar formalism: the backreaction of short-distance non-linearities and of
star-formation physics at long distances can be encapsulated in an effective
stress tensor, characterized by a few parameters. The functional form of
baryonic effects can therefore be predicted. In the power spectrum the leading
contribution goes as $\propto k^2 P(k)$, with $P(k)$ being the linear power
spectrum and with the numerical prefactor depending on the details of the
star-formation physics. We also perform the resummation of the contribution of
the long-wavelength displacements, allowing us to consistently predict the
effect of the relative motion of baryons and dark matter. We compare our
predictions with simulations that contain several implementations of baryonic
physics, finding percent agreement up to relatively high wavenumbers such as
$k\simeq 0.3\,h\, Mpc^{-1}$ or $k\simeq 0.6\, h\, Mpc^{-1}$, depending on the
order of the calculation. Our results open a novel way to understand baryonic
effects analytically, as well as to interface with simulations.
