  In the field of interacting electron systems the Mott metal-to-insulator (MI)
transition represents one of the pivotal issues. The role played by lattice
degrees of freedom for the Mott MI transition and the Mott criticality in a
variety of materials are current topics under debate. In this context,
molecular conductors of the $\kappa$-(BEDT-TTF)$_2$X type constitute a class of
materials for unraveling several aspects of the Mott physics. In this review,
we present a synopsis of literature results with focus on recent expansivity
measurements probing the Mott MI transition in this class of materials.
Progress in the description of the Mott critical behavior is also addressed.
