Project START (Students and Teachers Actively Reading Together) is an adaptive shared reading intervention designed to address the varied learning needs of preschool children with autism spectrum disorder (ASD). This report summarizes procedures and results of the developmental year of the project, which focused primarily on evaluating implementation fidelity and social validity of the intervention. The final sample consisted of four classrooms with 10 students with ASD (M[subscript age]= 4.32 years) and their teachers (N = 4). Classrooms were randomized to either a 4- or an 8-week first-stage small-group dialogic reading condition. Children who were early responders continued with the initial intervention; those who were slower to respond were randomized to one of two intensified reading conditions. Results indicate that teachers perceived the intervention as feasible and child outcomes as acceptable. Implementation fidelity was low during initial weeks (33%-50%), improving to 67% to 83% by the last weeks of the study. Neither children's engagement nor vocabulary growth differed between treatment levels or conditions. We discuss lessons learned from the study's developmental year and changes that will be made in subsequent years to improve implementation and feasibility. [This article was published in "Exceptional Children" (EJ1320422).]