Research over the past 50 years has yielded several promising approaches and many specific intervention techniques designed to enhance the communication and language development of young children with intellectual and developmental delays and disabilities. Yet virtually no systematic research has been conducted on the effects of different treatment intensities. We review how intervention intensity has been defined in the literature and propose a set of terms borrowed from medicine that are intended to capture the dynamic aspects of this concept as an aid to further investigation. On the basis of this approach, we propose four types of knowledge that can be generated through the systematic study of treatment intensity and discuss appropriate methods for investigating the effects of differential treatment intensities. We conclude with three recommendations for the field. © 2007 Wiley-Liss, Inc. MRDD Research Reviews 2007;13:70–77.