As a method of inquiry, single-case research has been relegated conventionally to the role of discovery or hypothesis generation. New developments in the methodology of intensive single-case designs have extended its applicability to the testing of clinical theoretical constructs and even the identification of causal relations. A series of articles illustrates newer developments in single-case research and its use in addressing a wide range of clinical and research questions. It has often been noted that psychotherapy research has had little influence on clinical practice. Single-case designs are more closely linked to traditional means of clinical inquiry, teaching, and learning than are large-sample studies, and they are likely to have more immediate relevance for how intervention is conducted.