Despite ongoing advances in the solution-based synthesis of inorganic nanocrystals, general methods that provide on-demand access to chemically complex nanostructures with controlled compositions, crystal structures, shapes, and sizes remain rare. We have been developing chemical reactions that transform pre-formed metal nanocrystal templates into more complex multi-element nanocrystals with compositions and morphologies that are defined by those of the template. In doing so, one can exploit advances in the shape-controlled synthesis of chemically simple nanocrystals (e.g. noble metals) and controllably transform them into more complex shape-controlled nanocrystal systems that are inaccessible using more direct synthesis methods. This talk will highlight recent advances in applying this "conversion chemistry" concept to the synthesis of morphology controlled intermetallic, alloy, oxide, sulfide, and phosphide nanocrystals, as well as advances in the size- and shape-controlled synthesis of metal nanocrystal templates such as Sn and In.