  We review dark energy models which can present non-negligible fluctuations on
scales smaller than Hubble radius. Both linear and nonlinear evolutions of dark
energy fluctuations are discussed. The linear evolution has a well-established
framework, based on linear perturbation theory in General Relativity, and is
well studied and implemented in numerical codes. We highlight the main results
from linear theory to explain how dark energy perturbations become important on
the scales of interest for structure formation. Next, we review some attempts
to understand the impact of clustering dark energy models in the nonlinear
regime, usually based on generalizations of the Spherical Collapse Model. We
critically discuss the proposed generalizations of the Spherical Collapse Model
that can treat clustering dark energy models and their shortcomings. Proposed
implementations of clustering dark energy models in halo mass functions are
reviewed. We also discuss some recent numerical simulations capable of treating
dark energy fluctuations. Finally, we make an overview of the observational
predictions based on these models.
