Thermodynamic aspects of solids are addressed as far as necessary for the discussion of ion transport and morphological metastability of nano-crystalline matter. Special emphasis is laid on the distinction between trivial and true size effects. As far as true size effects with respect to point defect chemistry are concerned, i.e. phenomena connected with changes of local properties, it is meaningful to differentiate between electrical size effects (space charge overlap) and structural size effects. Both influence the electrochemical potential, the former via the electrical potential term, the latter-which also includes elastic effects-via the non-configurational part of the chemical potential.