Participatory user interface design with adolescent users on the autism spectrum presents a number of unique challenges and opportunities. Through our work developing a system to help autistic adolescents learn to recognize facial expressions, we have learned valuable lessons about software and hardware design issues for this population. These lessons may also be helpful in assimilating iterative user input to customize technology for other populations with special needs. Keywords Autism, ASD, adolescents, design, user interface, emotion detection, facial expression analysis ACM Classification Keywords H5.2. User Interfaces: User-Centered Design.