Definitions of negative symptoms and deficit syndromes are reviewed in this article; rating scale criteria (the Scale for Assessment of Negative Symptoms and the Positive and Negative Syndrome Scale); Crow's type II syndrome; Carpenter's deficit syndrome; and the draft of DSM-IV. The core negative symptoms identified by these investigators also describe the neuroleptic-induced deficit syndrome (NIDS). Study designs and assessment methods to distinguish NIDS from the symptoms of schizophrenia are presented. Assessment methods include rating scales and a novel method that uses descriptive symptom vignettes. Designs include neuroleptic discontinuation studies, dose comparisons and comparison of atypical antipsychotic medicines to typical neuroleptic drugs.