The revised 3rd edition of the Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association, 1987) distinguishes between Axis I and Axis II disorders: Axis II includes personality (and developmental) disorders, and all others are on Axis I. This distinction is often useful, but the reification of Axis I and II constructs through diagnostic criteria sets that demarcate categorically distinct entities is at times problematic. We review the issues of differentiating personality from Axis I disorders, specifically illustrated by schizotypal and schizophrenic disorders, borderline and mood disorders, antisocial and substance use disorders, and avoidant personality from social phobia. The options for addressing their differentiation include adding exclusion criteria, shifting the placement of disorders, deleting overlapping criteria, adding differentiating criteria, and converting to a dimensional format.