Focused electron beam-induced deposition (FEBID) is capable of producing metal-containing nanostructures with lateral resolution on the sub-nanometer scale. Practical application of this nanofabrication technique has been hindered by ligand-derived contamination from precursors developed for thermal deposition methods. Mechanistic insight into FEBID through surface science studies and gas-phase electron–molecule interactions has begun to enable the design of custom FEBID precursors. These studies have shown that precursors designed to decompose under electron irradiation can produce high-purity FEBID deposits. Herein, we highlight the progress in FEBID precursor development with several examples that incorporate this mechanism-based design approach.