This chapter focuses on the concepts of element-blocks, single-component element-block materials, and polymeric materials based on element-blocks. These concepts have been proposed as a way to develop new materials that combine two independent fields: “organic–inorganic hybrid polymers” and “organic–inorganic hybrids.” A structural unit consisting of various group elements, including organic moiety as well as inorganic segments, is proposed and named “element-blocks.” In this section, carboranes, metal nanoparticles, and polyhedral oligomeric silsesquioxane (POSS) derivatives are discussed in this chapter as typical examples of element-blocks. Single-component element-block materials are attracting attention as a new approach to overcome the limitations of conventional hybrid preparation methods and achieve high concentrations and uniform dispersions beyond a certain level. The development of element-block monomers is necessary to create polymeric materials based on element-blocks. Carboranes and POSS derivatives are leading examples of element-block monomers with polymerizable functional groups. This chapter focuses on polymeric materials based on POSS element-blocks. This approach enables us to understand the structure–property relationship of silsesquioxane-based materials at the molecular level and design their properties more precisely than the conventional sol–gel method.