In August of 1991, a second Dartmouth International Workshop on the corpus callosum was convened to share and discuss the progress that had been made over the decade that had passed following the first workshop. A nucleus of basic and clinical scientists came together to discuss their work and the work of others in a field that has been broadened clinically by the addition of many new centers throughout the world that are now performing corpus callosotomy for intractable epilepsy. This text was stimulated by the participants' presentations and associated fertile discussions. It is compiled from the conference and subsequent studies. It reflects, both at the basic and clinical level, an important and expanding field of neural science endeavor. In keeping with the present and rapidly expanding field of outcomes assessment, callosotomy is again evaluated in light of a further decade of surgery and follow-up. Callosotomy continues to be a useful, palliative procedure and the indications for its use have been better established. The basic science section is a supplement to the first edition and elaborates progress in both new data and ideas. The section on experimental epilepsy models adds further support to the clinical rationale for callosotomy. Perhaps of greater importance is the contribution of experimental models to our understanding of the propagation of seizure activity. The section on the neuropsychology of the split brain patient demonstrates the continuing major contributions to the understanding of brain and behavior that pour forth from this cornucopia.