That genetic factors contribute to the etiology of schizophrenia is no longer debated; the nature and magnitude of that contribution, however, are still open for discussion. In this article, concordance rates for twin studies of schizophrenia are reviewed as one means of assessing the magnitude of the genetic contribution. Using only those studies in which representative samples were used and zygosity was determined with reasonable certainty, the pairwise concordance rate for schizophrenia was found to be 28 percent for monozygotic (MZ) and 6 percent for dizygotic (DZ) twins. Review of twin studies of other central nervous system diseases reveals that schizophrenia is most similar to multiple sclerosis (MZ concordance rate 27%). Although genetics remains as the single most clearly defined etiological factor in schizophrenia, the question remains whether we are overestimating the magnitude of the genetic contribution.