Many children born with severe and multiple disabilities have complex communication needs and may use no speech or only minimal speech to communicate. Meaningful assessment of their expressive skills to identify communication strengths along a developmental trajectory is an essential first step toward appropriate intervention. This article describes the foundations, structure, properties, and use of the "Communication Matrix," an assessment instrument developed specifically to address the challenges of describing the expressive communication skills of children with severe and multiple disabilities. The widely used online version of this assessment tool collects data in an associated database. Sample data on children with specific disabilities generated by this database are presented to illustrate the clinical and research potential of this free assessment service. (Contains 2 tables and 2 figures.)