We use experimental data to estimate impacts on school readiness of different kinds of preschool curricula – a largely neglected preschool input and measure of preschool quality. We find that the widely-used “whole-child” curricula found in most Head Start and pre-K classrooms produced higher classroom process quality than did locally-developed curricula, but failed to improve children's school readiness. A curriculum focused on building mathematics skills increased both classroom math activities and children's math achievement relative to the whole-child curricula. Similarly, curricula focused on literacy skills increased literacy achievement relative to whole-child curricula, despite failing to boost measured classroom process quality.