Lead halide perovskites are a sensational class of optoelectronic materials, but they suffer from the problems of lead toxicity and poor thermal stability of organic–inorganic hybrid systems. Various lead-free metal halide double-perovskite (DP) systems are being explored as potential lead-free perovskites for optoelectronics. In these DPs, Pb²⁺ are replaced by equal proportions of monovalent and trivalent metal ions. In early 2018, colloidal nanocrystals (NCs) of Cs₂AgBiX₆ (X = Cl, Br, I) DPs were reported and, in quick succession, multiple research teams have reported the synthesis, stabilization of unusual phases, optical properties, and photocatalytic applications of DP NCs. Here, we provide the first review of DP NCs. We first summarize the progress made in this research field, and then discuss the potential future directions of DP NCs compared with their bulk counterparts.