<p>‘High entropy’ has become a key concept in materials science over the past two decades, with this concept more recently extended to nanomaterials. High-entropy materials, characterized by the incorporation of five or more principal elements in nearly equal proportions, leverage entropy to promote the formation of compositionally complex single-phase materials rather than phase-segregated alternatives. The extensive compositional space of high-entropy nanomaterials, as well as their distinct structural and catalytic properties, has garnered considerable interest. The synthesis of high-quality single-phase high-entropy nanoparticles is important to fully realizing their potential to drive innovation, and numerous synthetic routes exist. Top-down methods begin with bulk high-entropy materials and break them down into nanosized structures, whereas bottom-up strategies start from atoms and build nanomaterials through nucleation and growth. In this Review, we categorize and compare the synthetic methods for high-entropy alloy and high-entropy intermetallic nanoparticles. Our discussion reveals that colloidal synthesis offers excellent control over the composition, size and shape of high-entropy nanoparticles while also providing pathways to metastable states that are not always accessible by other methods.</p>