The Diagnostic Guidelines Task Force of the International Society for Bipolar Disorders (ISBD) presents in this document and this special issue a summary of the current nosological status of bipolar illness, a discussion of possible revisions to current DSM-IV and ICD-10 definitions, an examination of the relevant literature, explication of areas of consensus and dissensus, and proposed definitions that might guide clinicians in the most valid approach to diagnosis of these conditions given the current state of our knowledge.