We have systematically investigated the production of “nanoemulsions,” droplets of one liquid phase in another immiscible liquid phase that have diameters less than 100 nm. Our approach relies on a combination of extreme shear due to multipass, high-pressure microfluidic injection and systematic control of the emulsion's composition. By repeatedly shearing a silicone oil-in-water emulsion in an inhomogeneous extensional shear flow, the multipass approach enables us to reduce the droplet polydispersity and average radius. Using dynamic light scattering, we study the changes in the average radius, 〈a〉, as a function of the number of passes, driving injection pressure (i.e., shear rate), droplet volume fraction, surfactant concentration, and droplet oil viscosity. The smallest nanoemulsion that we obtain has 〈a〉=18 nm. At large droplet volume fractions φ≥0.65, we observe phase inversion, rather than a reduction in the droplet size. This provides evidence that droplet coalescence can occur during extreme shear, even when a significant excess of a strongly stabilizing surfactant is present.