This issue elucidates that personality disorders as defined in the pathological range may be extrapolated into the normal field of personality traits. This is exemplified by an article explaining the factor structure based on questionnaires which relate to personality as well as to pathological disorders. The other four contributions deal with different biological systems, such as electrophysiological responses elicited by acoustically evoked potentials as connected to sensation seeking and affective disorders, the EEG alpha asymmetry related to depression in healthy and psychiatric patients, serotonergically induced endocrine responses in reference to harm avoidance, and finally, genetic polymorphisms of the serotonin transporter and catechol-O-methyltransferase and their association with Cloninger's trait of reward dependence. This issue addresses psychiatrists, pharmacologists, geneticists, psychologists to further their awareness that there is a continuum between psychiatric categories and psychological dimensions. Biological tests of reactivity either to pharmacological or physiological stimulation as well as genetic dispositions may be applied in both fields and yield converging information.