Metal-organic frameworks are a recently-identified class of porous polymeric material, consisting of metal ions linked together by organic bridging ligands, and are a new development on the interface between molecular coordination chemistry and materials science. A range of novel structures has been prepared which feature amongst the largest pores known for crystalline compounds, very high sorption capacities and complex sorption behaviour not seen in aluminosilicate zeolites. The development of synthetic approaches to these materials and investigations of their properties are reviewed.