We discuss to which extent the concepts of Zintl phases and of 'phonon glasses and electron single crystals' apply to type-I clathrates. In (β-) Eu 8 Ga 1 6 Ge 3 0 the presence of residual charge carriers appears to be related to a slight off-stoichiometry of the samples pointing to the validity of the Zintl concept in stoichiometric samples. The low and almost stoichiometry independent mobilities of (β-) Eu 8 Ga 1 6 Ge 3 0 , Sr 8 Ga 1 6 Ge 3 0 , and Ba 8 Ga 1 6 Ge 3 0 seriously question the validity of the 'electron single crystal' concept for type-I clathrates. The temperature dependence of the thermal conductivity of a Ba 8 Ga 1 6 Ge 3 0 single crystal indicates that tunneling states play a central role in producing 'phonon glass'-like thermal conductivities.