A fascinating aspect of many of the selenium- and tellurium-containing compounds described in this volume is that they are built up from octahedral Mo6 clusters which are surrounded by eight other atoms X in the form of a cube: Mo6X8. The Mo atoms occupy the centers of the faces of the X8 cube. The interesting variety in the substiution of the X positions can be encountered throughout the whole volume. The X atoms can be of a single element as in Mo6Te8, but can also be statistically occupied by two elements, e.g., in Mo6(Te,S)8, or even by more elements as in Mo6(Te,Se)8-yIy. In addition, there are channels with vacant positions between the Mo6X8 clusters in the diagonal direction of the X8 cube. These vacant positions offer more possibilities for the fantasy of the chemist.