Both suicidal and aggressive, impulsive behaviors have been linked to putative dysregulation in central serotonergic systems. We review data examining the role of serotonin (5-HT) in suicide from postmortem studies and clinical investigations of suicide attempters, including our own preliminary work derived from neuroendocrine challenges with the 5-HT uptake inhibitor clomipramine. Various approaches to the study of 5-HT and aggressive, impulsive behavior, including cerebrospinal fluid studies, investigations of peripheral measures of 5-HT, and neuroendocrine studies utilizing 5-HT probes, are highlighted. Several important caveats, including the challenge of quantifying "suicidality" and "aggression" in reliable and valid ways, should be considered in interpreting the results of clinical studies of 5-HT and suicide and aggression.