Porous materials have long since been a popular area of research with significant impacts on society both in terms of economic and social change. Traditionally, the most applicable of these porous materials were purely inorganic silicates (zeolites) and the porous amorphous organic compounds, such as porous carbons and polymers. The most recent advances in the field of porous materials are metal organic compounds, combining organic linkages with metal clusters. These metal organic composites can be built to form discrete species in the form of metal-organic polyhedra or polygons (MOPs) or they can form interconnected lattices producing metal-organic frameworks (MOFs). This chapter aims to highlight the importance of porous molecular materials. By focusing on the history, structure, synthesis and applications of MOPs, we intend to emphasise the significance of this novel family of materials, which transcend coordination chemistry and molecular solid-state chemistry. This chapter and the references provided within are a starting point for a more in depth look at the world of porosity within molecular solids and will focus on the relationship between intrinsic and extrinsic porosity.