This article summarizes research focusing on communication skills of people with intellectual and developmental disabilities (IDD) who communicate primarily without speech, sign language, or augmentative communication. Importantly, developmental theories were emphasized as useful for interpreting communication in children and adults. These studies led to research aimed at developing improved assessments and interventions for individuals with IDD, who may have a variety of diagnoses. Future research is needed to facilitate more widespread use of these tools by clinicians and researchers.