ConspectusNanoparticles that contain multiple materials connected through interfaces, often called heterostructured nanoparticles, are important constructs for many current and emerging applications. Such particles combine semiconductors, metals, insulators, catalysts, magnets, and other functional components that interact synergistically to enable applications in areas that include energy, nanomedicine, nanophotonics, photocatalysis, and active matter. To synthesize heterostructured nanoparticles, it is important to control all of the property-defining features of individual nanoparticles-size, shape, uniformity, crystal structure, composition, surface chemistry, and dispersibility-in addition to interfaces, asymmetry, and spatial organization, which facilitate communication among the constituent materials and enable their synergistic functions. While it is challenging to control all of these nanoscale features simultaneously, nanoparticle cation exchange reactions offer powerful capabilities that overcome many of the synthetic bottlenecks. In these reactions, which are often carried out on metal chalcogenide materials such as roxbyite copper sulfide (Cu1.8S) that have high cation mobilities and a high density of vacancies, cations from solution replace cations in the nanoparticle. Replacing only a fraction of the cations can produce phase-segregated products having internal interfaces, i.e., heterostructured nanoparticles. By the use of multiple partial cation exchange reactions, multicomponent heterostructured nanoparticles can be synthesized.In this Account, we discuss the use of multiple sequential partial cation exchange reactions to rationally construct complex heterostructured nanoparticles toward the goal of made-to-order synthesis. Sequential partial exchange of the Cu+ cations in roxbyite Cu1.8S spheres, rods, and plates produces a library of 47 derivatives that maintain the size, shape, and uniformity defined by the roxbyite templates while introducing various types of interfaces and different materials into the resulting heterostructured nanoparticles. When an excess of the metal salt reagent is used, the reaction time controls the extent of partial cation exchange. When a substoichiometric amount of metal salt reagent is used instead, the extent of partial cation exchange can be precisely controlled by the cation concentration. This approach allows significant control over the number, order, and location of partial cation exchange reactions. Up to seven sequential partial cation exchange reactions can be applied to roxbyite Cu1.8S nanorods to produce derivative heterostructured nanorods containing as many as six different materials, eight internal interfaces, and 11 segments, i.e. ZnS-CuInS2-CuGaS2-CoS-[CdS-(ZnS-CuInS2)]-Cu1.8S. We considered all possible injection sequences of five cations (Zn2+, Cd2+, Co2+, In3+, Ga3+) applied to all accessible Cu1.8S-derived nanorod precursors along with simple design criteria based on preferred cation exchange locations and crystal structure relationships. Using these guidelines, we mapped out synthetically feasible pathways to 65 520 distinct heterostructured nanorods, experimentally observed 113 members of this heterostructured nanorod megalibrary, and then made three of these in high yield and in isolatable quantities. By expansion of these capabilities into a broader scope of materials and identification of additional design guidelines, it should be possible to move beyond model systems and access functional targets rationally and retrosynthetically. Overall, the ability to access large libraries of complex heterostructured nanoparticles in a made-to-order manner is an important step toward bridging the gap between design and synthesis.