Motivated as a possible unified description for dark matter and dark
energy, we analyze the cosmological applications and the stability
conditions of Mimetic Gravity models. In connection to the fact that
these models have been recently shown to be plagued by instabilities, we
show that for the original mimetic matter model the ghost/tachyon instability
for perturbations around the FLRW background can be interpreted as the
standard Jeans instability of dust.
Proposed as a solution for the instability problems, we discuss mimetic
gravity theories containing higher derivatives of the scalar field coupled
to the curvature. In particular, we analyze the effect on the number of
degrees of freedom if we identify the time flow with the scalar field.