This chapter addresses the complex inter relationships between anxiety disorders (ADs) and personality disorders (PDs), including diagnostic controversies, patterns of comorbidity, and prognostic implications for treatment. The Diagnostic and Statistical Manual of Mental Disorders (DSM-III) created five axes for diagnostic classification, including Axis I for illnesses such as ADs and Axis II for PDs. The presence of an Axis I state disorder may confound diagnosis of Axis II trait. The biopsychosocial model suggests that illness arises from the interplay of genetic, trait, and environmental influences. PDs are less prevalent than ADs. Limited information exists on the comorbidity between ADs and PDs in the general population. Available data suggest that PD comorbidity among ADs is common, except in the case of specific phobia. Current evidence yields more questions than answers, due in part to the complexity of assessing PDs and the labor and expense of large-scale, longitudinal studies.