The author reviews research strategies for identifying biological vulnerability traits and gene markers. The application of these methods to schizophrenia are discussed. He concludes that, based on the available evidence, none of the proposed biological traits qualifies as a major factor in the genetic susceptibility to schizophrenia. Some of these traits (such as attentional, neurophysiological, and brain morphology measures) appear promising, but the data are inadequate to arrive at a firm conclusion. Studies with gene markers have not produced consistent results. The author presents the methodological issues that concern this area of research and offers recommendations for future studies.