In chapter 3, Hwang and associates address the problem of comorbid obsessive-compulsive disorder (OCD) in schizophrenia. The prevalence of the phenomenon (1% to 6% of patients with schizophrenia have comorbid OCD symptoms during the course of their illness) is quite high and deserves careful assessment, particularly when the obsessive symptoms overlap with delusional phenomena, leading to a complex and difficult-to-disentangle clinical picture. The authors focus on the relative onset of the two phenomena and identify three groups of patients with schizophrenia and OCD—i.e., those whose OCD symptoms preceded the onset of schizophrenia, those with simultaneous onsets of schizophrenia and OCD, and those whose OCD symptoms developed after the onset of schizophrenia. These three groups show clear differences in clinical course, but overall their outcome is worse than that of patients with schizophrenia who do not have comorbid OCD. The problems related to treatment management are also discussed. For example, the addition of a serotonergic antidepressant with antiobsessional activity may interfere with the antipsychotic treatment.