Like atoms or molecules, nanocrystals can be used as the building blocks of condensed matter. Assembling nanocrystals into solids opens up the possibilities of fabricating new solid-state materials and devices with novel or enhanced physical and chemical properties, as interactions between proximal nanocrystals give rise to new collective phenomena. Many different synthesis routes to semiconductor, noble metal, magnetic metal alloy and oxide nanocrystals are readily available, providing good control over particle size, shape, size distribution, composition and surface properties. Tailoring of the properties of the final material is made easily possible by chosing the appropriate nanoparticulate building block.