  A review is given on the theoretical studies of charge correlations in
$\theta$-(BEDT-TTF)$_2X$. Various studies show that within a purely electronic
model on the $\theta$-type lattice with the on-site $U$ and the nearest
neighbor $V_p$ and $V_c$ interactions, the diagonal stripe, c-axis three-fold,
and the vertical stripe charge correlations are favored in the regime $V_p<
V_c$, $V_p\sim V_c$, and $V_p> V_c$, respectively. In the realistic parameter
regime of $V_p\sim V_c$, there is a competition between c-axis three fold state
and the diagonal stripe state. Since these are different from the
experimentally observed a-axis three fold and the horizontal stripe charge
correlations, additional effects have to be included in order to understand the
experiments. The electron-lattice coupling, which tends to distort the lattice
into the $\theta_d$-type, is found to favor the horizontal stripe state,
suggesting that the occurrence of this stripe ordering in the actual materials
may not be of purely electronic origin. On the other hand, distant
electron-electron interactions have to be considered in order to understand the
a-axis three fold correlation, whose wave vector is close to the nesting vector
of the Fermi surface. These studies seem to suggest that the minimal model to
understand the charge correlation in $\theta$-(BEDT-TTF)$_2X$ may be more
complicated than expected. Future problems regarding the competition between
different types of charge correlations are discussed.
