This introduction to the Journal of Personality's special issue on the topic of defense mechanisms raises the question of why defenses have been largely overlooked in personality research. Although defenses were originally discussed in the context of psychopathology, for the past 60 years psychodynamically oriented psychologists have understood that defenses play an important part in normal psychological development. Recently, psychologists outside the field of personality have incorporated the ideas of unconscious mental processes and of defenses into their research programs. In this issue, personality researchers discuss their ideas about defense mechanisms, organized around three topical areas: current conceptualizations of defense, the measurement of defense mechanisms, and the integration of defense mechanisms into personality research. The issue concludes with a commentary on these articles and provides suggestions for future work.